This periodically test the connection with a MySQL native ping before sending a command.

This is a passive way to reconnect, without losing a query.
I manually applied Eonblast/Emysql pull request #45 to refresh the patch.
See: https://github.com/Eonblast/Emysql/pull/54
Update emysql.erl

add a guard clause to add_pool/8
Conflicts:
	src/emysql.erl
Databases can be `undefined`

The environment suite supplies an `undefined` atom as the database parameter.
Alter the code to allow for this.
Fix test cases to comply with connection checking.

@mremond introduced a patch which does periodical connection checking to
make sure Mysql connections are up and running. Alter the test cases to
take into account that `#emysql_connection{}` records now have more fields.