encoding/json: get rid of the stream_test.go TODOs

TestRawMessage now passes without the need for the RawMessage field to
be a pointer. The TODO dates all the way back to 2010, so I presume the
issue has since been fixed.

TestNullRawMessage tested the decoding of a JSON null into a
*RawMessage. The existing behavior was correct, but for the sake of
completeness a non-pointer RawMessage field has been added too. The
non-pointer field behaves differently, as one can read in the docs:

	To unmarshal JSON into a value implementing the Unmarshaler
	interface, Unmarshal calls that value's UnmarshalJSON method,
	including when the input is a JSON null.

Change-Id: Iabaed75d4ed10ea427d135ee1b80c6e6b83b2e6e
Reviewed-on: https://go-review.googlesource.com/131377


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>