Fix WSP folding before attribute values

Some mailservers do a line folding within the Content-Type parameters (see below or the tests).

For example:
> boundary=
>     "=_0c5bb6a1"

[RFC2822](http://tools.ietf.org/html/rfc2822#section-2.2.3) mentions that a folding after higher-level syntactic breaks is allowed. The "=" within attributes seems to be interpreted as such as well.

The fix removes optional whitespaces after the equal sign.