Updated fix for button element and added test cases

lib/kramdown/parser/html.rb

@@ -33,7 +33,7 @@ module Constants
         HTML_TAG_CLOSE_RE = /<\/(#{REXML::Parsers::BaseParser::UNAME_STR})\s*>/m
         HTML_ENTITY_RE = /&([\w:][\-\w\.:]*);|&#(\d+);|&\#x([0-9a-fA-F]+);/
 
-        HTML_CONTENT_MODEL_BLOCK = %w{address applet article aside button blockquote body
+        HTML_CONTENT_MODEL_BLOCK = %w{address applet article aside blockquote body
              dd details div dl fieldset figure figcaption footer form header hgroup iframe li map menu nav
               noscript object section summary td}
         HTML_CONTENT_MODEL_SPAN  = %w{a abbr acronym b bdo big button cite caption del dfn dt em

test/testcases/span/05_html/button.html

@@ -0,0 +1,7 @@
+<p><button>Some Text</button></p>
+
+<p>First some text and then a <button>here</button></p>
+
+<p><button>A button</button> and then text.</p>
+
+<p>A <button>with <strong>some</strong> <code>code</code> in <img src="image.png" alt="alt" /></button> it.</p>

test/testcases/span/05_html/button.text

@@ -0,0 +1,7 @@
+<button>Some Text</button>
+
+First some text and then a <button>here</button>
+
+<button>A button</button> and then text.
+
+A <button>with **some** `code` in ![alt](image.png)</button> it.