http: HttpClient: add ClearCallback()

Added to prevent HttpClient from calling a registered callback method. Useful
ie. when you'd like to abandon the request.

Change-Id: If5a8147f155b704515ea6b052100da2e54e331d8
Signed-off-by: Bartlomiej Wolowiec <bartlomiej.wolowiec@codilime.com>
Signed-off-by: Mateusz Zalega <mateusz.zalega@codilime.com>
Merge "http: HttpClient: add ClearCallback()"