Pass parameters to promote/rollback as querystring

[lennym]

Custom build params are ignored when sent on the POST body and need to be attached to the querystring. Modify the promote and rollback functions to ensure that the params are sent in a way that is recognised by the Drone server.

[lennym]

Note: I haven't been able to update any of the unit tests since it appears the existing test suite runs against an actual live Drone instance, so it is not reasonable to test build promotions with parameters in this way.

[mtandre]

I've recently run into this bug of params not being passed to the promote command via this lib.

Hey @lennym would you be up for making the small adjustment mentioned above in this PR? If not, no worries, I'd be happy to open another PR with the fix.