Support for multiline headers about http-parser

[mnot]

Just FYI, HTTPbis is discouraging the use of multi-line headers; from http://tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-11#section-3.2 :

Historically, HTTP header field values could be extended over
multiple lines by preceding each extra line with at least one space
or horizontal tab character (line folding). This specification
deprecates such line folding except within the message/http media
type (Section 10.3.1). HTTP/1.1 senders MUST NOT produce messages
that include line folding (i.e., that contain any field-content that
matches the obs-fold rule) unless the message is intended for
packaging within the message/http media type. HTTP/1.1 recipients
SHOULD accept line folding and replace any embedded obs-fold
whitespace with a single SP prior to interpreting the field value or
forwarding the message downstream.

I.e., the parser still SHOULD be able to handle them, but the spec now admits they're not widely supported.