Method ::Mail::AttachmentsList#[]= clobbers hash assigned

Mail::AttachmentsList#[]= , given a hash object to assign (instead of a literal hash), clobbers it (by deleting its key-value pairs). (Here is a tiny example [gist](https://gist.github.com/MarkDBlackwell/7897447).)

Clobbering this hash object in this assignment seems to violate [POLS](http://en.wikipedia.org/wiki/Principle_of_least_astonishment) (the Principle Of Least Surprise).

This problem IMHO could be avoided best here (on [this line](https://github.com/mikel/mail/blob/8e455b910607d439e1b369df0d66de8e3f7f707e/lib/mail/attachments_list.rb#L46)) by adding:

value = value.dup

Start at lib/mail/attachments_list.rb, specifically the line 46 assignment described in the issue. Verify the behavior with a hash object rather than a literal hash, then ensure the assigned hash is not modified while the attachment list receives the expected value.

Written by the indexing model from the issue text.