Fix DefaultEncoder to handle Array-of-pairs input

Add encode_pairs to handle top-level Array input as key-value pairs,
while preserving [] notation for nested array values. Update the test
expectation to reflect the new encoder's [] array encoding.

Author: sferik

lib/http/form_data/urlencoded.rb

@@ -70,7 +70,11 @@ def encode(value, prefix = nil)
               when Hash
                 encode_hash(value, prefix)
               when Array
-                value.map { |v| encode(v, "#{prefix}[]") }.join("&")
+                if prefix
+                  value.map { |v| encode(v, "#{prefix}[]") }.join("&")
+                else
+                  encode_pairs(value)
+                end
               when nil then prefix.to_s
               else
                 raise ArgumentError, "value must be a Hash" if prefix.nil?
@@ -80,6 +84,10 @@ def encode(value, prefix = nil)
 
             private
 
+            def encode_pairs(pairs)
+              pairs.map { |k, v| encode(v, escape(k)) }.reject(&:empty?).join("&")
+            end
+
             def encode_hash(hash, prefix)
               hash.map do |k, v|
                 encode(v, prefix ? "#{prefix}[#{escape(k)}]" : escape(k))

spec/lib/http/form_data/urlencoded_spec.rb

@@ -7,7 +7,7 @@
   it "supports any Enumerables of pairs" do
     form_data = described_class.new([%w[foo bar], ["foo", %w[baz moo]]])
 
-    expect(form_data.to_s).to eq("foo=bar&foo=baz&foo=moo")
+    expect(form_data.to_s).to eq("foo=bar&foo[]=baz&foo[]=moo")
   end
 
   describe "#content_type" do