Just like HAML tends to do, filters content must be rstrip

This avoids the bloc of code being showing extra blank lines at the end

CHANGELOG.md

@@ -1,3 +1,5 @@
+- Fixed HAML :code filter to remove trailing blank lines
+
 # 3.2.0
 
 - Prep for Middleman v5

features/haml_filter.feature

@@ -11,3 +11,15 @@ Feature: Haml :code filter.
     When I go to "/code_haml_filter.html"
     Then I should see '<span class="k">def</span>'
     Then I should see '<pre class="highlight plaintext"><code>This is some code'
+
+  Scenario: Filter doesn't keep trailing blank lines
+    Given a fixture app "test-app"
+    And a file named "config.rb" with:
+      """
+      set :haml, { :ugly => false }
+      activate :syntax
+      """
+    Given the Server is running at "test-app"
+    When I go to "/code_haml_filter.html"
+    Then I should see '<span class="k">def</span>'
+    Then I should see '<pre class="highlight plaintext"><code>This is some code</code></pre>'

lib/middleman-syntax/haml_monkey_patch.rb

@@ -6,6 +6,7 @@ module Code
         include Base
 
         def render(code)
+          code = code.rstrip
           code = code.encode(Encoding::UTF_8)
 
           # Allow language to be specified via a special comment like: