Get ~100% test coverage for ApiEditPage.php and fix a couple of bugs

Get ~100% test coverage for ApiEditPage.php and fix a couple of bugs

Bugs fixed:

* The major one was that appendtext and prependtext seemingly didn't
  work at all on empty pages in the MediaWiki namespace, because
  $this->getTitle() was being used in place of $titleObj.
* Negative undo/undoafter were not rejected in a correct fashion.
* If a logged-in user who was not allowed to upload images tried to
  create an image redirect, the error message used a nonexistent key.

I also replaced assertEquals with assertSame.

I also removed a bit of clearly dead code.  This includes a "break"
following dieStatus().  If we actually want this break so that nobody
adds a case after the default and then removes the dieStatus() so the
switch incorrectly falls through, it could be re-added with
@codeCoverageIgnore.

I put the fixes in the same commit as the test changes because I like to
keep fixes together with the tests for those fixes.

All code now shows up as covered locally, except for one line that seems
to be a PHPUnit bug.

Change-Id: I9375bc5f40268fd681a2d447c66a03f40b23390a