For instance:
HttpRequest.newBuilder().method("foo", null)
does not throw IllegalArgumentException.

Maybe the spec should also say whether "CONNECT" should be accepted or not...

Also the spec is probably missing some @throws in HttpRequest.Builder.build(), and the implementation might be missing
some validation as well:
  - HttpRequest.Builder.build() doesn't throw any exception if method is null
  - HttpRequest.Builder.build() throws NPE if uri is null.