parser: Ensure whitespace is stripped for long headers

parser: Ensure whitespace is stripped for long headers

RFC2822 states that long headers can be wrapped using CRLF followed by
WSP [1]. For example:

    Subject: Foo bar,
     baz

Should be parsed as:

    Foo bar,baz

While we were stripping the former, we were not stripping the
latter. This mean that we ended up with the following:

    Foo bar, baz

Resolve this.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Closes: #197
(cherry picked from commit 841f966b8d54b2f51ab1c498eed6e5391f2546a9)