do not allow markdown etc. in excerpt

Adjust the tests accordingly. Also
add a message for one of the tests.

lib/jekyll/post.rb

@@ -89,7 +89,11 @@ module Jekyll
     #
     # Returns excerpt string.
     def excerpt
-      self.data['excerpt'] ? converter.convert(self.data['excerpt']) : self.extracted_excerpt
+      if self.data.has_key? 'excerpt'
+       self.data['excerpt']
+      else
+       self.extracted_excerpt
+      end
     end
 
     # Compares Post objects. First compares the Post date. If the dates are

test/source/_posts/2013-04-11-custom-excerpt.markdown

@@ -1,6 +1,6 @@
 ---
 layout: ~
-excerpt: 'I can set a custom excerpt with *markdown*'
+excerpt: 'I can set a custom excerpt'
 ---
 
 This is not my excerpt.

test/test_post.rb

@@ -329,11 +329,11 @@ class TestPost < Test::Unit::TestCase
           end
 
           should "use custom excerpt" do
-            assert_equal("<p>I can set a custom excerpt with <em>markdown</em></p>", @post.excerpt)
+            assert_equal("I can set a custom excerpt", @post.excerpt)
           end
 
           should "expose custom excerpt to liquid" do
-            assert @post.content.include?("I can use the excerpt: <quote><p>I can set a custom excerpt with <em>markdown</em></p></quote>")
+            assert @post.content.include?("I can use the excerpt: <quote>I can set a custom excerpt</quote>"), "Exposes incorrect excerpt to liquid."
           end
 
         end