Fix MySQL dialect

[adrien-berchet]

MySQL dialect is not completely working, as shown in this test: https://github.com/geoalchemy/geoalchemy2/blob/master/tests/test_functional.py#L436
This probably points a deep issue in MySQL support so we have to understand why it fails and fix it.

[satyamsoni2211]

@adrien-berchet I think my fix should fix this issue as well.

[adrien-berchet]

I tried to rerun the failing test with the new release and unfortunately it still fails (this test: tests/test_functional.py::TestInsertionORM::test_transform[mysql]).