http://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.5
Specification of Request-Header fields:
http://www.w3.org/Protocols/rfc2616/rfc2616-sec5.html#sec5.3
Specification of Entity-Header fields:
http://www.w3.org/Protocols/rfc2616/rfc2616-sec7.html#sec7.1
For the definition of the categories the standard request message header fields are of
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HTTP request message parser
Before parsing an HTTP request message, the names of all header fields whose values will be required must be added by calling addCustomHeaders, except the General-Header and Request-Header fields specified in RFC 2616 section 4.5 and 5.3, respectively. After parse() has finished parsing the message hader, the values of these message header fields of interest can be obtained by the ParamSet (HttpRequest super class) methods. A null value indicates that the request message does not contain a header line whose name matches the corresponding key. Specification of General-Header fields: