Parsing with header value token checks off.
GET /url HTTP/1.1
Header1: \f
off=0 message begin
off=4 len=4 span[url]="/url"
off=19 len=7 span[header_field]="Header1"
off=28 len=1 span[header_value]="\f"
off=33 headers complete method=1 v=1/1 flags=100 content_length=0
off=33 message complete
GET /url HTTP/1.1
Header1: \f
off=0 message begin
off=4 len=4 span[url]="/url"
off=19 len=7 span[header_field]="Header1"
off=28 error code=10 reason="Invalid header value char"