[SPARK-2890][SQL] Allow reading of data when case insensitive resolution could cause possible ambiguity.

Throwing an error in the constructor makes it possible to run queries, even when there is no actual ambiguity.  Remove this check in favor of throwing an error in analysis when they query is actually is ambiguous.

Also took the opportunity to add test cases that would have caught a subtle bug in my first attempt at fixing this and refactor some other test code.