Handle parsing of LF-only body with separate parts

[sebbASF]

Conversion of body to CRLF does not help here because body is not ascii_only so does not get converted. Instead to search for CR?LF (optional CR) in which case conversion to CRLF is unnecessary.

[jeremy]

Thanks @sebbASF! Dig this.

[akissinger]

I ran into this error myself, and nearly made the same PR (without the nice unit tests of course...). Is this going to get merged at some point?

[mikel]

Waiting for specs to be working on the master forward port of this.

[sebbASF]

Redundant as 2.7 no longer in development