This prevents later errors like the one reported in #23324 and throws an
IllegalArgumentException early instead.

* master:
  [TEST]  make headers case-insensitive when running yaml tests
  [TEST] randomize request content_type between all of the supported formats
  [TEST] add support for binary responses to REST tests infra
  [TEST] don't check exact size in mapper-size yaml test
  [TEST] move test for binary field to specific test file that sets Content-Type header explicitly
  [TEST] move filters aggs wrapper query builder rewriting test to integ tests
  [TEST] create HttpEntity earlier in REST tests
  [TEST] Remove content type auto-detection while parsing request body in REST tests
  Factor out filling of TopDocs in SearchPhaseController (elastic#23380)
  Add info method to High Level Rest client (elastic#23350)
  Prevent negative `from` parameter in SearchSourceBuilder (elastic#23358)
  reduce the number of iterations in testPrimaryRelocationWhileIndexing and flush every 5
  rollback unneeded change in testNotifyOnDisconnect
  disable sampling in testNotifyOnDisconnect