Update plugin/gnupg.vim

Need to use copy() when setting b:GPGRecipients in GPGDecrypt(), otherwise the local variable will act as a reference to the global one.  The result will be that the global default recipient list will become a super-set of recipients as multiple gpg files are opened.
Merge pull request #6 from matt-garman/patch-1

copy() g:GPGDefaultRecipients to b:GPGRecipients