api: fix swh.core.api.decode_request on python 3.5
The json decoder was broken on py35, as is used to expect a str object and not a bytes one. For now, we encode the bytes one using utf-8.
This could/should be reverted as soon as we drop support for py35.
Test Plan
this should fix the tests on jenkins
Migrated from D2126 (view on Phabricator)