Do not collapse multiple quoted-printable lines if the line encodings are not the same.

If you have multiple lines where one has an encoding of Q and the other has an encoding of B, the existing logic would collapse these lines together. This change makes it so that only lines with the same encoding are collapsed.
Merge pull request #480 from adamvaughan/encoding-bug

Do not collapse multiple quoted-printable lines if the line encodings are not the same.