When we set the payload, it is converted to bytes using the response's
character set. Changing the MIME type later has no effect on the conversion
to bytes, even though it does change the returned MIME type. This results in
bytes that were encoded using one character set while the response reports a
different character set.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=196258752
c25f765fc5