default deployment ends up with invalid value of query-cache-size

Deploy mysql with default value.

From the charm config.yaml:

   query-cache-size:
        default: -1
        type: int
        description: Override the computed version from dataset-size. Still works if query-cache-type is "OFF" since sessions can override the cache type setting on their own.

The resulting /etc/mysql/my.cnf contains:
query_cache_size = -1

According to http://dev.mysql.com/doc/refman/5.1/en/server-system-variables.html#sysvar_query_cache_size
query_cache_size should be a positive integer ranging from 0 to 4294967295

The charm should not write a negative digit there.