Fix caching of MIME config

* in get_mimes(): was missing isset() test
* in Email->_mimes_types(): static cache of reference was noneffective

refs 6ef498b49946ba74d610b3805fb908b163a7f03a
Remove static cache in Email->_mimes_types()

Saving one function call isn't worth the code overhead.
Merge pull request #3052 from vlakoff/mime

Fix caching of MIME config