I had the same issue when we started to use Hibernate. Assing additional configuration to the connection URL didn't solve the problem, however we did add the following properties
<property name="hibernate.connection.characterEncoding">utf8</property>
<property name="hibernate.connection.useUnicode">true</property>
<property name="hibernate.connection.charSet">UTF-8</property>
and that what solve the problem. Try removing parameters from URL string and leave it be so it would be just like
jdbc:mysql://localhost:3306/db_name
I hope it will solve your problem.
EDIT: I just noticed that you are not using Hibernate at all so this will not help you ;/