CachingHttpClient should have similar behavior as AbstractHttpClient when executing with ResponseHandler

When calling execute on the AbstractHttpClient with a ResponseHandler, the AbstractHttpClient will attempt to Consume the Entity and close any open connections before returning. This behavior is not currently in the CachingHttpClient.

This can lead to connection leaks when switching to CachingHttpClient, becuase the responsibility to fully consume the entity is now on the ResponseHandler instead on the HttpClient.

Here is the code that does the existing 'auto-close' behavior: "org.apache.http.impl.client.AbstractHttpClient.java" lines 1080-1111