refactored MultiGetIntegrationTest for readability and more case coverage
[BREAKING] refactored MultiGet action and corresponding request helper class Doc:
 - moved doc-to-json-string conversion code from MultiGet to Doc
 - [BREAKING] removed unnecessary public setters and modifiers from Doc
refactored MultiGet action and corresponding request helper class Doc:
 - offloaded doc-to-json-string conversion to Gson (instead of using string builder)
 - modified MultiGet constructor hierarchy to use a common hidden constructor
 - changed ById builder's super class to multi index/type supporting one to play nice with uri setting mechanism (eventhough ES does not support multi index/type for MultiGet requests - possible pitfall)