Add test cases for checking content decoding (#791)

Author: mingkang111

tests/unitary/with_extras/aqua/test_data/valid_eval_artifact/report.html

@@ -1 +1,2 @@
 This is a sample evaluation report.html.
+Standard deviation (σ)

tests/unitary/with_extras/aqua/test_evaluation.py

@@ -624,9 +624,10 @@ def test_download_report(
         mock_dsc_model_from_id.assert_called_with(TestDataset.EVAL_ID)
         self.print_expected_response(response, "DOWNLOAD REPORT")
         self.assert_payload(response, AquaEvalReport)
-        read_content = base64.b64decode(response.content)
+        read_content = base64.b64decode(response.content).decode()
         assert (
-            read_content == b"This is a sample evaluation report.html.\n"
+            read_content
+            == "This is a sample evaluation report.html.\nStandard deviation (σ)\n"
         ), read_content
         assert self.app._report_cache.currsize == 1