html: handle '<' before a tag

As pointed out at
https://groups.google.com/forum/#!topic/golang-nuts/LJozHIXAAJY,
`<<p>html</p>` was parsed as `&lt;&lt;p&gt;html</p>`.
There was no test case for this. Chrome parses it as `&lt<p>html</p>`,
and that seems to be correct. We were missing the
"Reconcume the current input character" step at
http://www.whatwg.org/specs/web-apps/current-work/multipage/tokenization.html#tag-open-state

LGTM=nigeltao
R=golang-codereviews, gobot, nigeltao
CC=golang-codereviews, nigeltao
https://golang.org/cl/96060044

html/token.go

@@ -1002,6 +1002,8 @@ loop:
 			// "<!DOCTYPE declarations>" and "<?xml processing instructions?>".
 			tokenType = CommentToken
 		default:
+			// Reconsume the current character.
+			z.raw.end--
 			continue
 		}
 

html/token_test.go

@@ -105,6 +105,11 @@ var tokenTests = []tokenTest{
 		"if x<0 and y < 0 then x*y>0",
 		"if x&lt;0 and y &lt; 0 then x*y&gt;0",
 	},
+	{
+		"not a tag #11",
+		"<<p>",
+		"&lt;$<p>",
+	},
 	// EOF in a tag name.
 	{
 		"tag name eof #0",