Fixed issue with StartContainerCmd

The accept header was being set incorrectly to text instead of json.
Also added a defensive null check in JsonClientFilter.
Fixed startContainer use cases

Using top command instead of true so that the container keeps running
and returns the valid state
Fixed issue with StartContainerCmd

The accept header was being set incorrectly to text instead of json.
Also added a defensive null check in JsonClientFilter.
Merge branch 'master' of https://github.com/NirmataOSS/docker-java

Conflicts:

src/test/java/com/github/dockerjava/client/command/StartContainerCmdTest
.java
Fix merge issue

Fix merge issue
Merge pull request #22 from NirmataOSS/master

Fixes for startContainerCmd