Accept header lines with only spaces

RFC 5322 defines "The body is simply a sequence of characters that
follows the header section and is separated from the header section by
an empty line (i.e., a line with nothing preceding the CRLF)."

So theoretically it is valid to have a header line with only spaces, and
some clients do this.
Merge pull request #1 from bcronje/master

Accept header lines with only spaces