Tests for starting a Container with existing configuration

This illustrates an issue with current docker-java: you cannot send a
StartContainerCmd without payload.
Do not serialize unconfigured values in StartContainerCmd

Any payload sent with the StartContainerCmd will cause the target
container to lose its customization.
So in order to just start a preconfigured container you must be able to
send an empty "{}" message to the /containers/{id}/start endpoint.

This is accomplished by all configuration defaulting to null in
combination with @JsonInclude(NON_EMPTY).
Merge pull request #111 from albers/empty-start-command

Allow to send empty messages in StartContainerCmd