Include separating whitespace when folding multi-line header values

Support for folding of multi-line header values was added to
http-parser a couple of years ago.  But the implementation does not
conform to the specs.  Given a request containing

    Multi-Line-Header: foo<CRLF>
     bar<CRLF>

http-parser will eliminate the whitespace breaking the header value to
yield a header value of "foobar".  This is confirmed by the
LINE_FOLDING_IN_HEADER case in tests.c.

But from rfc2616, section 2.2:

   A CRLF is allowed in the definition of TEXT only as part of a header
   field continuation. It is expected that the folding LWS will be
   replaced with a single SP before interpretation of the TEXT value.

And from draft-ietf-httpbis-p1-messaging-25, section 3.2.4:

   A server that receives an obs-fold in a request message that is not
   within a message/http container MUST either reject the message by
   sending a 400 (Bad Request), preferably with a representation
   explaining that obsolete line folding is unacceptable, or replace
   each received obs-fold with one or more SP octets prior to
   interpreting the field value or forwarding the message downstream.

So in the example above, the header value should be interpreted as
"foo bar", possibly with multiple spaces.  The current http-parser
behaviour of eliminating the LWS altogether clearly deviates from the
specs.

For http-parser itself to confirm exactly would involve significant
changes in order to synthesize replacement SP octets.  Such changes
are unlikely to be worth it to support what is an obscure and
deprecated feature.  But http-parser should at least preserve some
separating whitespace when folding multi-line header values, so that
applications using http-parser can conform to the specs.

This commit is a minimal change to preserve whitespace when folding
lines.  It eliminates the CRLF, but retains any trailing and leading
whitespace in the header value.
Fix issues around multi-line headers

Always discard leading whitespace in a header value, even if it
is folded.

Pay attention to values of interesting headers (Connection,
Content-Length, etc.) even when they come on a continuation line.

Add a test case to check that requests and responses using only LF
to separate lines are handled correctly.