Rework boundary parsing for the multipart body (fix #137 ) (#139 )

* Rename C++ exported functions (add cpp_ prefix)

* Change boundary generation for the tests

* Fix boundary parsing and matching

* Add echo demo app

* Add test for the Request$set_id

* Wrap BackendRserve$start with nocov

* Fill Request content-type from headers only if empty (fix tests fail)

* Use ASCII to generate boundary for the tests

* Simulate Rserve behaviour for multipart body in test

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