CF-154: handle mixed content, add tests

This PR will allow publisher to send the following:
```
"content" : {
  "type" : "application/json",
  "@text": {
   // arbitrary json
  }
}
```
This will be converted to an atom content like this:
```
<atom:content type="application/json">
{
   // arbitrary json
 }
</atom:content>
```
This will allow current Cloud Feeds publishers to continue sending arbitrary JSON payload. We allow this for events that will never be exposed to external customers.
We should also add the isAtomShortHand( String prefix, BidiMap nsPrefixMap ) check to this condition to only execute this in the atom namespace.