Properly handle `case_sensitive?` for mysql

[ronen]

ActiveRecord's mysql adapter provides a case_sensitive? method, which checks the column collation type. By a lucky accident, schema_plus_columns doesn't actually override the mysql method -- see @dmeranda's analysis in SchemaPlus/schema_validations#38,

But the code, specs, and README should make explicit that mysql's native method is respected. In fact the index-based hack used for postgresql should be documented explicitly as a hack since no native method is available.