RFC2231 filename continuations of Content-Disposition header.

This patch allows you to appropriately deal with filename continuations in Content-Disposition header.
The params continualtion is defined in RFC 2231(http://tools.ietf.org/html/rfc2231).
When specifying a long filename as a attachment file, a string of the filename will be sometimes splitted into several pieces, like 'filename*0*=' and  'filename*1*='.

Requirements: email.utils
If the length of the list is ```1```, there must be no filename continuation. So the value of the variable ```filename``` doesn't need to be changed.