Correctly detect obsolete header line folding

[GeorgeLyon]

From https://tools.ietf.org/html/rfc7230#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.

Picohttpparser currently treats header fields with "\r\n\t" in the value as two header fields. For instance GET /hoge HTTP/1.1\r\nHost: ex\r\n\tample.com\r\nCookie: \r\n\r\nsplits the Host header field.

The correct behavior would be to return a parsing error (-1)