Make possible to provide a specific HTTP request object to File

This will be used in a following commit to test how the X-OC-MTime
header is handled.

This commit is based on the "make File::put() more testable" commit
(included in 018d45cad97e0) from ownCloud by Artur Neumann.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>