TST: enable MySQL tests

PR to fix and enable the MySQL tests for the new sql functionality.

- floats (decimal.Decimal) were not converted due to a small bug in the `coerce_float` arg. This is now OK.
- MySQL has no real BOOL type (it is just an alias for tiny int). So I changed the tests for that (so a bool column is converted to int64 (or float if there are NA's)

We should maybe add somewhere in the sql docs an overview of the limitations of the type conversion.