SDK / exoplayer

DataSpec: Make DataSpecTest more useful

- Replace tests specific to HTTP request headers with tests that
  more generally test constructor correctness.
- Add test specifically around the constructor that infers the
  HTTP method from postBody, since that's a non-trivial bit of
  constructor logic.

Note: In general, I'm not sure this type of test (i.e., testing a
bit of trivial copying through a constructor) is worth the cost
of writing and maintaining them. That said, checking the new
uriPositionOffset is calculated correctly is non-trivial, as is
the HTTP method inference, so keeping them for now.
PiperOrigin-RevId: 294488684