sdk: Don't add parameters with null values

Currently the methods that receive header or URL parameters add them
to the request even if the value is null. This patch changes those
methods to not add the header or URL parameter in this case.

Change-Id: Ifabf318632e57302727683d78355f1805b48b6af
Bug-Url: https://bugzilla.redhat.com/1047638
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>