Better logic / handle scoping in case_sensitive?

[ronen]

schema_plus_columns' case_sensitive? postgresql implementation returns true if any index that the column is involved in is case sensitive. That isn't always what you want; see the discussion in the commit message of @dmeranda's commit 89f54f to SchemaPlus/schema_validations#39.

Perhaps case_sensitive? should take an optional argument specifying a scope. (If so, mysql should likewise accept a scope argument that it would ignore -- see #1) If a scope were provided, it could be used to obviate the extra logic in @dmeranda's commit 89f54f.

And should the default behavior (no scope specified) change to be something more conservative? That'd be a breaking change of course.