Stop to join B-encoded lines

Hello.
It looks to me as though 'mail' have a bug in its decoding argorithm for B-encodeed string.

Q-encoded strings can be able to join before decode them.
Otherwise, B-encoded strings do not hold this property.

However, B-encoded strings were wrongly joined
in Mail::Encodings.collapse_adjacent_encodings.

If my notion is not wrong.
Could you merge my fix?

thanks.