Revert "Revert "Double quote stings that contain single quotes only""

Author: hsbt

lib/psych/visitors/yaml_tree.rb

@@ -304,7 +304,7 @@ def visit_String o
           quote = false
         elsif @line_width && o.length > @line_width
           style = Nodes::Scalar::FOLDED
-        elsif o =~ /^[^[:word:]][^"]*$/
+        elsif o =~ /^[^[:word:]][^"]*$/ or o =~ /^([^"]*'+[^"]*)+$/
           style = Nodes::Scalar::DOUBLE_QUOTED
         elsif not String === @ss.tokenize(o) or /\A0[0-7]*[89]/ =~ o
           style = Nodes::Scalar::SINGLE_QUOTED

test/psych/test_string.rb

@@ -37,6 +37,13 @@ def test_doublequotes_when_there_is_a_single
       assert_equal str, Psych.load(yaml)
     end
 
+    def test_doublequotes_when_there_are_single_quotes_only
+      str = "psych: Please don't escape ' with ' here."
+      yaml = Psych.dump str
+      assert_equal "--- \"psych: Please don't escape ' with ' here.\"\n", yaml
+      assert_equal str, Psych.load(yaml)
+    end
+
     def test_plain_when_shorten_than_line_width_and_no_final_line_break
       str = "Lorem ipsum"
       yaml = Psych.dump str, line_width: 12