Keep headerScanner strict so malformed MIME header lines are still rejected.
Move trimming before ':' into the HTTP header handling paths that
intentionally normalize header names, and add a fuzz seed for the
regression case.
This change updates header parsing to match the behavior of net/http more closely.
**Breaking change**: headers delimited by `\n` (instead of `\r\n`) are no longer supported.
Previously, fasthttp accepted `\n` as a delimiter, which is not spec compliant.
This made it difficult to correctly parse headers containing both `\n` and `\r\n`.
Erik Dubbelboer
RW