Make async match sync when saving dynamic objects

This is my first ever pull request, so please be kind.  I appreciate all tips where I messed up on proper protocol.  

This fix fixes http://issues.hibernatingrhinos.com/issue/RavenDB-2192, when saving dynamic objects using async session, the behavior does not match the sync behavior.  The sync follows the documentation that the HiLo generator is used to generate an id on the client.  But Async does not generate this id and the id defaults to a new sequential Guid.