HTTP headers are a method for client and the server to share additional
information that will help with web request.
Typically proprietary headers start with the X- prefix.
However, this convention was deprecated in June 2012 because of the inconveniences
it caused when nonstandard fields became standard in
RFC 6648; others are
listed in an IANA registry,
whose original content was defined in RFC 4229.
IANA also maintains a registry of
proposed new HTTP headers.
An HTTP header consists of its case-insensitive name followed by a colon (:),
then by its value. Whitespace before the value is ignored.