Removing conditional type coercion for zset.add

[vincentamoore]

Redis represents zset scores as floats internally, but mock_redis attempts to coerce zset scores to integers. This coercion prevents certain float values from being used as scores (eg. Float::INFINITY).

This PR removes the conditional type coercion from the zset add method, and instead converts all scores to floats.

[sds]

Thank you!