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.
Merge pull request #681 from srawlins/allocate-less-things

Allocate less things

Conflicts:
	lib/mail/fields/common/common_field.rb
	lib/mail/utilities.rb
autoload parsers for load-time speed and memory usage