HttpRequestPtr reset interface

[bethebest0622]

trafficstars

if i want to reuse the existed HttpRequestPtr.

I need to clear the header (the previous request needs header, but current request dont need).

I found there is no clearHeader interface.

RemoveHeader is not effectient, could you add this?

[hwc0919]

Just a reminder, the HttpRequest object should only be reused after the previous request finished (technically it is safe to reuse after the previous request is sent, but we don't have a notification point there).