Fix whitespace trimming in headings

Fix whitespace trimming in headings

* b3dd3881 was trimming whitespace in wikitext as well as HTML headings
  whereas the whitespace-trimming proposal was going to leave HTML tags
  untouched.

* 30495ea1 missed this because coincidentally, the test I added there
  for HTML headings had a typo and used <h2>...<h2> instead of
  <h2>...</h2> which caused the test to magically pass.

* This patch trims whitespace in
  doHeadings (which deals with wikitext headings) instead of
  formatHeadings (which deals with all headings).

* Updated parser tests to account for this.

Change-Id: I854f20b4c39a0a8e03d70155b269de77acf02cae