Updated for mime-type 2.0.

The extra dependency of mime-types has been updated to use the 2.0 version of that gem. This is largely because many test frameworks that might incorporate Mechanize (like Cucumber) use mime-types 2.0. What would happen is that errors like this would occur:

Unable to activate mechanize-2.7.2, because mime-types-2.0 conflicts with mime-types (>= 1.17.2, ~> 1.17) (Gem::LoadError)

Also, even with the previous mime-type a deprecated usage of MIME::InvalidContentType was being used. I updated that to MIME::Type::InvalidContentType.