Correctly detect modified tokens (bug from commit 46119efb).

* The wikitext: "[a = {{echo|b}}]" was not being properly parsed
  and consequently RT-ed because AttributeExpander was ignoring
  modified tokens (from expanded attributes).

* Fixed and verified.

* The page en:Gigabarrel from which this test case was extracted
  now RTs correctly.

* No change in parser test results -- worth adding a new test.

Change-Id: I71ecf966e6dad9ed874f2cdfb4a8a5d91ec86d2f
Merge "Correctly detect modified tokens (bug from commit 46119efb)."