Prevent result cache key collisions when sharing cache across different connections

There's an issue when using the default cache key generation for the result cache and using the same cache system across different connections as the generated key will be the same regardless of the connection used.

We can solve this just by using the connection params in the key generation. This issue is quite similar to the one fixed in [#1075](https://github.com/doctrine/doctrine2/pull/1075).
No actual need to check the expected cache key: what is relevant is that for same connection params, the same cache key is built, and for different connection params the opposite. As it stands, the test is a bit fragile