Prohibit fruitless modification of httpBuildRequestHeader flags parameter.

Prohibit fruitless modification of httpBuildRequestHeader flags parameter.

HttpStateData::httpBuildRequestHeader is a static method, but it has a
parameter called flags, just like HttpStateData objects have a data member
called flags. Modifying that parameter "worked" but had no effect on the
caller's flags. Wasted a few good hours.

The parameter is "const" now, to prevent fruitless modification.

Also removed http_state_flags parameter from HttpStateData::buildRequestPrefix
which is not a static method and has access to the "real" flags member.

No runtime effect expected.

TODO: Rename HttpStateData::httpBuildRequestHeader to mark its static nature.
Does it belong to HttpStateData at all?