fix: XPath following, following-sibling, preceding, preceding-sibling for multiple node

## Why?
See: #251 (comment)

- XPath : a/d/preceding::* => ["d", "c", "b"]
```xml
<a>
  <b/> <!-- a/d/preceding::b -->
  <c/> <!-- a/d/preceding::c -->
  <d/> <!-- a/d/preceding::d -->
  <d/> <!-- self -->
  <e/>
  <f/>
</a>
```

- XPath : a/d/following::* => ["d", "e", "f"]
```xml
<a>
  <b/>
  <c/>
  <d/> <!-- self -->
  <d/> <!-- a/d/following::d -->
  <e/> <!-- a/d/following::e -->
  <f/> <!-- a/d/following::f -->
</a>
```

- XPath : a/b/x/following-sibling:* => ["c", "d", "e"]
```xml
<a>
  <b>
    <x/> <!-- self -->
    <c/> <!-- a/b/x/following-sibling::c -->
    <d/> <!-- a/b/x/following-sibling::d -->
 </b>
  <b>
    <x/> <!-- self -->
    <e/> <!-- a/b/x/following-sibling::e -->
  </b>
</a>
```

- XPath : a/b/x/following-sibling:* => ["c", "d", "x", "e"]
```xml
<a>
  <b>
    <x/> <!-- self -->
    <c/> <!-- a/b/x/following-sibling::c -->
    <d/> <!-- a/b/x/following-sibling::d -->
    <x/> <!-- a/b/x/following-sibling::x -->
    <e/> <!-- a/b/x/following-sibling::e -->
  </b>
</a>
```

- XPath : a/b/x/preceding-sibling::* => ["e", "d", "c"]
```xml
<a>
  <b>
    <c/>  <!-- a/b/x/preceding-sibling::c -->
    <d/>  <!-- a/b/x/preceding-sibling::d -->
    <x/>  <!-- self -->
  </b>
  <b>
    <e/>  <!-- a/b/x/preceding-sibling::e -->
    <x/>  <!-- self -->
  </b>
</a>
```

- XPath : a/b/x/preceding-sibling::* => ["e", "x", "d", "c"]
```xml
<a>
  <b>
    <c/>  <!-- a/b/x/preceding-sibling::c -->
    <d/>  <!-- a/b/x/preceding-sibling::d -->
    <x/>  <!-- a/b/x/preceding-sibling::x -->
    <e/>  <!-- a/b/x/preceding-sibling::e -->
    <x/>  <!-- self -->
  </b>
</a>
```

Author: naitoh

lib/rexml/xpath_parser.rb

@@ -297,6 +297,7 @@ def expr( path_stack, nodeset, context=nil )
             descendant(nodeset, false)
           end
         when :following_sibling
+          nodeset = nodeset.group_by{|node| node.raw_node.parent}.values.map{|nodes| nodes.min_by{|node| node.position}}
           nodeset = step(path_stack) do
             nodesets = []
             nodeset.each do |node|
@@ -314,6 +315,7 @@ def expr( path_stack, nodeset, context=nil )
             nodesets
           end
         when :preceding_sibling
+          nodeset = nodeset.group_by{|node| node.raw_node.parent}.values.map{|nodes| nodes.max_by{|node| node.position}}
           nodeset = step(path_stack, order: :reverse) do
             nodesets = []
             nodeset.each do |node|
@@ -331,12 +333,14 @@ def expr( path_stack, nodeset, context=nil )
             nodesets
           end
         when :preceding
+          nodeset = nodeset.max_by(1){|node| node.position}
           nodeset = step(path_stack, order: :reverse) do
             unnode(nodeset) do |node|
               preceding(node)
             end
           end
         when :following
+          nodeset = nodeset.min_by(1){|node| node.position}
           nodeset = step(path_stack) do
             unnode(nodeset) do |node|
               following(node)

test/xpath/test_base.rb

@@ -416,10 +416,36 @@ def test_preceding
       assert_equal( 4, cs.length )
     end
 
+    def test_preceding_multiple
+      d = REXML::Document.new("<a><b/><c/><d/><d/><e/><f/></a>")
+      matches = REXML::XPath.match(d, "a/d/preceding::node()")
+      assert_equal(["d", "c", "b"], matches.map(&:name))
+    end
+
+    def test_following_multiple
+      d = REXML::Document.new("<a><b/><c/><d/><d/><e/><f/></a>")
+      matches = REXML::XPath.match(d, "a/d/following::node()")
+      assert_equal(["d", "e", "f"], matches.map(&:name))
+    end
+
+    def test_following_sibling
+      d = REXML::Document.new("<a><b><x/><c/><d/></b><b><x/><e/></b></a>")
+      matches = REXML::XPath.match(d, "a/b/x/following-sibling::node()")
+      assert_equal(["c", "d", "e"], matches.map(&:name))
+
+      d = REXML::Document.new("<a><b><x/><c/><d/><x/><e/></b></a>")
+      matches = REXML::XPath.match(d, "a/b/x/following-sibling::node()")
+      assert_equal(["c", "d", "x", "e"], matches.map(&:name))
+    end
+
     def test_preceding_sibling
       d = REXML::Document.new("<a><b><c/><d/><x/></b><b><e/><x/></b></a>")
       matches = REXML::XPath.match(d, "a/b/x/preceding-sibling::node()")
       assert_equal(["e", "d", "c"], matches.map(&:name))
+
+      d = REXML::Document.new("<a><b><c/><d/><x/><e/><x/></b></a>")
+      matches = REXML::XPath.match(d, "a/b/x/preceding-sibling::node()")
+      assert_equal(["e", "x", "d", "c"], matches.map(&:name))
     end
 
     def test_following