Avoid unnecessary replacements when the node doesn't change

This represents a +2x performance optimization when the replacement
logic is based on some condition, and it returns the same unchanged
node when it wants to skip it:

 ```ruby
 html = <<~HTML
       <div>
         #{'<p>ignore me</p>' * 1000}
         #{'<p>replace me</p>' * 1000}
       </div>
 HTML

 content = content_from_html(html)

 replacement_example = -> do
   content.fragment.replace("p") do |node|
     if node.text =~ /replace me/
       "<p>replace me</p>"
     else
       node
     end
   end
 end

 current_implementation = -> do
   class ActionText::Fragment
     def replace(selector)
       update do |source|
         source.css(selector).each do |node|
           replacement_node = yield(node)
           node.replace(replacement_node.to_s)
         end
       end
     end
   end

   replacement_example.call
 end

 new_implementation = -> do
   class ActionText::Fragment
     def replace(selector)
       update do |source|
         source.css(selector).each do |node|
           replacement_node = yield(node)
           node.replace(replacement_node.to_s) if node != replacement_node
         end
       end
     end
   end

   replacement_example.call
 end

 Benchmark.ips do |x|
   x.report "Current implementation", &current_implementation
   x.report "New implementation", &new_implementation

   x.compare!
 end
 ```

 Results:

 ```
 Warming up --------------------------------------
 Current implementation
                          2.000  i/100ms
   New implementation     5.000  i/100ms
 Calculating -------------------------------------
   Current implementation
                          32.484  (±30.8%) i/s -    134.000  in   5.036419s
   New implementation     74.878  (±38.7%) i/s -    250.000  in   5.052168s

 Comparison:
   New implementation:       74.9 i/s
   Current implementation:       32.5 i/s - 2.31x  (± 0.00) slower
 ```

Author: jorgemanrubia

actiontext/lib/action_text/fragment.rb

@@ -37,7 +37,8 @@ def update
     def replace(selector)
       update do |source|
         source.css(selector).each do |node|
-          node.replace(yield(node).to_s)
+          replacement_node = yield(node)
+          node.replace(replacement_node.to_s) if node != replacement_node
         end
       end
     end

actiontext/test/unit/content_test.rb

@@ -134,6 +134,33 @@ class ActionText::ContentTest < ActiveSupport::TestCase
     assert_match %r/\A#{Regexp.escape '<div class="trix-content">'}/, rendered
   end
 
+  test "replace certain nodes" do
+    html = <<~HTML
+      <div>
+        <p>replace me</p>
+        <p>ignore me</p>
+      </div>
+    HTML
+
+    expected_html = <<~HTML
+      <div>
+        <p>replaced</p>
+        <p>ignore me</p>
+      </div>
+    HTML
+
+    content = content_from_html(html)
+    replaced_fragment = content.fragment.replace("p") do |node|
+      if node.text =~ /replace me/
+        "<p>replaced</p>"
+      else
+        node
+      end
+    end
+
+    assert_equal expected_html.strip, replaced_fragment.to_html
+  end
+
   private
     def content_from_html(html)
       ActionText::Content.new(html).tap do |content|