Avoid `useless use of == in avoid context` warning.

…by calling a method on the result.
It's a hack but it'll help us to keep rspec-expectations warning free.
Lock mime types to version that works on 1.8.
Merge pull request #346 from rspec/avoid-uselss-use-in-void-context-warning

Avoid `useless use of == in avoid context` warning.