[viz] Fix payload force logic

[john-bodley]

The logic to force fetch the visualization payload is specified via the force request argument rather than the force key in the form-data.

Note the form-data serves as the cache key and thus if this logic were correct the force field would need to be removed from cache_key, i.e., the key should be agnostic of forcing function.

[coveralls]

Coverage decreased (-0.004%) to 71.513% when pulling 95b6515 on john-bodley:john-bodley-fix-payload-force into 4d48d5d on apache:master.

[mistercrunch]

Yeah that's an artifact of the era where form_data was the actual query string as opposed to serialized json