[PATCH] Proper assignment and copying of HttpHeader

Code cleanup: Proper assignment and copying of HttpHeader.

Besides being the Right Thing, having correct assignment operator and
copy constructor helps classes that have HttpHeader data members to
avoid defining explicit assignment operators and copy constructors.

Also adds forgotten reset of "len" in the clean() method. Perhaps that
data member is not needed at all?

Other polishing touches. HttpHeader::reset() is now a tiny bit faster.

No runtime changes expected.

Thank you,

Alex.
P.S. I stumbled upon this ancient mess while enhancing adaptation
History classes. This patch will remove a few dozen lines from those
classes, but those changes will be submitted separately.