Merge of the folding-reimpl-branch. Specific changes,

Merge of the folding-reimpl-branch.  Specific changes,

_handle_multipart(): Ensure that if the preamble exists but does not
end in a newline, a newline is still added.  Without this, the
boundary separator will end up on the preamble line, breaking the MIME
structure.

_make_boundary(): Handle differences in the decimal point character
based on the locale.