Fix double escaped backtrace links in TextMateFormatter.

This was introduced in https://github.com/rspec/rspec-core/commit/b295e165658c0cc2f72a14d8c666a552d4b9b489

The regression went unnoticed because the spec “produces HTML identical to the one we designed manually” was flawed: It did some sanity checking for each backtrace link, but it failed to fail if there were no links at all. Which exactly is what happened when the double escape was introduced: the backtrace did no longer contain any (unescaped) links.
Merge pull request #1356 from noniq/fix-double-escape-in-textmate-formatter

Fix double escaped backtrace links in TextMateFormatter.