Bug fix in trivial special case in token-and-attr-collector.

* In wikitext like this: {{echo|<math>a=b</math>}}, the tokenizer
  parses this as:

  {"type":"SelfclosingTagTk",
   "name":"template",
   "attribs":[{"k":"echo","v":""},
              {"k":[{"type":"TagTk","name":"math","attribs":[]},"a"],
               "v":["b",{"type":"EndTagTk","name":"math","attribs":[]}]}
             ]
  }

  The <math> and </math> tags end up in the attributes of the template.
  TokenAndAttrCollector attempts to merge this into a single array of
  tokens.  But, it didn't correctly handle the special/trivial case 
  where the opening and closing tags ended up in the same attribute (in 
  this case, the second attribute).

  This patch fixes this.

  Note that this bug does not do anything bad for this code snippet,
  or for other examples probably, but it does lead to duplicate content
  in the merged token (evident in debug/trace mode), and could possibly
  cause problems on other non-html tags that show in tpl-arg positions 
  (I couldn't easily come up with  an example where parsed/RT output is 
  fixed by this patch).

Change-Id: I6789439cd3629aca6002ab964fad5582ae48a928