[SPARK-8356] [SQL] Reconcile callUDF and callUdf

Deprecates ```callUdf``` in favor of ```callUDF```.

Author: BenFradet <benjamin.fradet@gmail.com>

Closes #6902 from BenFradet/SPARK-8356 and squashes the following commits:

ef4e9d8 [BenFradet] deprecated callUDF, use udf instead
9b1de4d [BenFradet] reinstated unit test for the deprecated callUdf
cbd80a5 [BenFradet] deprecated callUdf in favor of callUDF