assertions.py: Fix mis-leading assert_is_json

Parameter name "a_string" was mistaken and function
name "assert_is_json" was less clear than ideal,
given that it explicitly needs bytes unlike json.loads .

tests/assertions.py

@@ -11,9 +11,10 @@ def assert_cassette_has_one_response(cass):
     assert cass.play_count == 1
 
 
-def assert_is_json(a_string):
+def assert_is_json_bytes(b: bytes):
+    assert isinstance(b, bytes)
     try:
-        json.loads(a_string.decode("utf-8"))
+        json.loads(b.decode("utf-8"))
     except Exception:
         assert False
     assert True