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

[MarkDBlackwell]

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.)

Clobbering this hash object in this assignment seems to violate POLS (the Principle Of Least Surprise).

This problem IMHO could be avoided best here (on this line) by adding:

value = value.dup

[jeremy]

Still an issue on master. We should not be mutating arguments, for sure.