Stop quote_phrase from modifying input string encoding, fixing #557.

In the case where it acted to quote the input string, quote_phrase shouldn't modify the encoding of the input string to be ASCII-8BIT, but should leave the encoding as it found it.

This fixes both https://github.com/mikel/mail/issues/557 and https://github.com/mikel/mail/issues/569.

https://github.com/mikel/mail/pull/575 unfortunately isn't the perfect fix, as it adds an unnecessary dup. This change is cleaner.