Added ApiHeaderProperties allowing users to pass any header (as long as it is supported) to a method. This will be useful for situations in the future where the API adds new headers and we have not had the time to add named properties/parameters for these new headers in the driver.
Added stream transaction support in the Collection and Graph APIs
Nice idea to add support for additional headers. I realise it looks quite trivial but wonder if we should consider having one or mote simple unit tests for ToWebHeaderCollection method?
Solves issue #328