Added get filename as encoded
 This filename described in RFC 6266
Added test for encoded filename
Removed assert for wrong charset
Removed use encoding available only in py3
Refactor encoded filename feature per #1531
Fix encoded filename parsing to allow for lang
Move parser tests to correct directory
Refactor disposition unpacking for clarity
Merge pull request #1838 from jbittel/fix-encoded-filename-rfc6266

Support RFC6266 encoded filenames