Add support for sql_mode and fix set charset

[ruifil]

• Add the support for sql_mode https://dev.mysql.com/doc/refman/8.0/en/sql-mode.html
• The character set must be applied in all cases if it is defined, since we don't know the server's default character set.

[xpader]

I think getCharset() should never return null, cause there always have charset setted, from client set or server default, if we not set charset in client, getCharset() may return a value that auto detected.

[ruifil]

I think getCharset() should never return null, cause there always have charset setted, from client set or server default, if we not set charset in client, getCharset() may return a value that auto detected.

I don't agree due to the fact getCharset() is a method of the connection parameter bag, if is empty don't change the database charset on database connection.
In order to know the database default connection charset, we can do show variables like "character_set_client"; after connection.

An alternative solution is to set the charset to utf8mb4/utf8mb4_general_ci but call SET NAMES in any case on each database connection.

[kelunik]

It should likely be utf8mb4_0900_ai_ci and always call set names of course, but then we'd probably need fallback for older versions of mysql? So maybe it's better to really default to the connection default set by the server?