session

Replaced the usage of undocumented `res._header` with `res.headersSent` (regression-safe)

Added test cases to check for `res._header` and `res.headersSent` equivalence

Removed explicit http2 headers for less verbosity (using default values)

Replaced the usage of undocumented http API with the proper writeHead method (bringing about http2 support)

Added test cases (http2) that will fail if using undocumented `res._implicitHeader()`