fix errant uses of safe_cast

[drewbanin]

• do not use safe_cast when inappropriate (see here) (union_tables, unpivot)
• improve union_tables integration test
• fix union_tables for Snowflake
• add unpivot integration test -- this was disabled before, which was terrible

Note that this changes the behavior of unpivot slightly. Duplicate column names with different casing are no longer permitted, but I think that's a fair assumption to make.

[jthandy]

nice--this is great. I'm all for it.

I should really add some documentation around safe_cast and its intended usage. I kind of doubt I'll get to that soon but it would be good to avoid this same misunderstanding down the road.