Display of user Fields broken

#368559: Make profile pages return renderable arrays broke the display of Fields on user pages. No need to even add fields to users, you currently get warnings just by going to user/1
Simple fix, I dare say it's RTBC.

This outlines that we currently don't have tests for fields as being attached to actual entities (node, users...). Field tests stay entity-agnostic by attaching to a 'test_entity'. Not sure how we should handle that. We don't want to duplicate existing tests for each fieldable entity...
Leaving this to a separate issue, though.