No-escape underscores in MySQL table/column identifiers

Underscore characters `_` in table/column/database identifiers should not be backslash-escaped, as they're in the character class MySQL & SQLite allow for unquoted identifiers, and would not be treated as wildcards in a `SELECT` statement like they would in a `GRANT` statement.

http://dev.mysql.com/doc/refman/5.0/en/identifiers.html