This should fix issue #559, for example:
  Content-Type: multipart/alternative;
    boundary=mk3-ebfa33c1cd454cc2b1c618f5d74b41af; charset=UTF-8

We were assuming all boundary parameter values are quoted (apparently not
true, see https://www.rfc-editor.org/rfc/rfc2046#section-5.1.1) - so support
non-quoted boundary values which also may have a semicolon delimiter.

Also, truncate any white-space from a boundary value (shouldn't be there per
RFC2046).