Narrowing conditions for which we defensively dup-unparent-and-root t…

…ext nodes in the expectation they may be merged. Closes #595.

Note that this simply omits the case where we are inserting a previous
text sibling, and our case was duping and unparenting the *next* text
sibling.

It's not totally clear to me, based on looking at the libxml2 code,
that this is necessary if the pivot node is not a text node. That is,
we may be being *too* defensive here.

We should probably schedule some work to rethink all the kludges we
have around node reparenting, because I'm pretty sure we can simplify
(or decompose) `reparent_node_with` with a little thought.

ext/nokogiri/xml_node.c

@@ -138,7 +138,7 @@ static VALUE reparent_node_with(VALUE pivot_obj, VALUE reparentee_obj, pivot_rep
     }
   }
 
-  if (reparentee->type == XML_TEXT_NODE && pivot->next && pivot->next->type == XML_TEXT_NODE) {
+  if (prf != xmlAddPrevSibling && reparentee->type == XML_TEXT_NODE && pivot->next && pivot->next->type == XML_TEXT_NODE) {
     /*
      *  libxml merges text nodes in a right-to-left fashion, meaning that if
      *  there are two text nodes who would be adjacent, the right (or following,

test/xml/test_node_reparenting.rb

@@ -246,6 +246,22 @@ class TestNodeReparenting < Nokogiri::TestCase
             replacee.add_previous_sibling "foo <p></p> bar"
             assert_equal "foo <p></p> bartext node", xml.root.children.to_html
           end
+
+          describe "with a text node before" do
+            it "should not defensively dup the 'before' text node" do
+              xml = Nokogiri::XML %Q(<root>before<p></p>after</root>)
+              pivot  = xml.at_css("p")
+              before = xml.root.children.first
+              after  = xml.root.children.last
+              pivot.add_previous_sibling("x")
+
+              assert_equal "after", after.content
+              assert !after.parent.nil?, "unrelated node should not be affected"
+
+              assert_equal "before", before.content
+              assert !before.parent.nil?, "no need to reparent"
+            end
+          end
         end
 
         describe "#add_next_sibling" do
@@ -255,6 +271,22 @@ class TestNodeReparenting < Nokogiri::TestCase
             replacee.add_next_sibling "foo <p></p> bar"
             assert_equal "text nodefoo <p></p> bar", xml.root.children.to_html
           end
+
+          describe "with a text node after" do
+            it "should defensively dup the 'after' text node and not touch the 'before' text node" do
+              xml = Nokogiri::XML %Q(<root>before<p></p>after</root>)
+              pivot  = xml.at_css("p")
+              before = xml.root.children.first
+              after  = xml.root.children.last
+              pivot.add_next_sibling("x")
+
+              assert_equal "before", before.content
+              assert !before.parent.nil?, "unrelated node should not be affected"
+
+              assert_equal "after", after.content
+              assert_nil  after.parent
+            end
+          end
         end
 
         describe "#replace" do