Fix #4: nested, indirect descendant, <button> causes infinite loop.

A couple of elements (button, dialog) were missing from the list of
endTagBlock in-body-phase dispatcher. This adds them.

See html5lib/html5lib-tests#4 for test.

Author: gsnedders

html5lib/html5parser.py

@@ -926,10 +926,10 @@ def __init__(self, parser, tree):
             self.endTagHandler = utils.MethodDispatcher([
                 ("body", self.endTagBody),
                 ("html", self.endTagHtml),
-                (("address", "article", "aside", "blockquote", "center",
-                  "details", "dir", "div", "dl", "fieldset", "figcaption", "figure",
-                  "footer", "header", "hgroup", "listing", "main", "menu", "nav", "ol",
-                  "pre", "section", "summary", "ul"), self.endTagBlock),
+                (("address", "article", "aside", "blockquote", "button", "center",
+                  "details", "dialog", "dir", "div", "dl", "fieldset", "figcaption", "figure",
+                  "footer", "header", "hgroup", "listing", "main", "menu", "nav", "ol", "pre",
+                  "section", "summary", "ul"), self.endTagBlock),
                 ("form", self.endTagForm),
                 ("p", self.endTagP),
                 (("dd", "dt", "li"), self.endTagListItem),