let me set a default charset
Cope with raw sources that were read with the wrong encoding.

Force raw message source to binary encoding before doing CRLF
conversion, masking common issues when users read a message using the
default external encoding (UTF-8), resulting in invalid UTF-8 characters
that break the regexp used for CRLF conversion.

Fixes #736.
Fix 1.8.7 regression in 1ab7d7b5442e0b7a44838725dc623866f4f99fde

Array#sort_by! was added in Ruby 1.9.2.
Merge pull request #761 from zendesk/grosser/default-char

Expose `Mail::Message.default_charset = ...` to set the default charset for new messages. Defaults to `'UTF-8'`
simplify encoding splitter and support whitespace inside encoded