http://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.5
Specification of Request-Header fields:
http://www.w3.org/Protocols/rfc2616/rfc2616-sec6.html#sec6.2
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 response message generator
Before rendering an HTTP response message, the names of all header fields the response may contain must be added, except the General-Header, Response-Header and Entity-Header fields specified in RFC 2616 section 4.5, 6.2 and 7.1, respectively. Before calling render(), the values of these message header fields of interest can be assigned by the ParamSet (HttpResponse super class) methods. Header fields with a null value (which is the value reset() assigns to all fields) will be omitted when rendering the response message header. Specification of General-Header fields: