Test that Blackd also skips first line correctly

Simliarly to the Black tests, we first compare that Blackd fails when
the first line is invalid Python syntax and then check that the result
is the expected when tha flag is activated

Signed-off-by: Antonio Ossa Guerra <[email protected]>

tests/test_blackd.py

@@ -177,6 +177,20 @@ async def test_blackd_invalid_line_length(self) -> None:
         )
         self.assertEqual(response.status, 400)
 
+    @unittest_run_loop
+    async def test_blackd_skip_first_source_line(self) -> None:
+        invalid_first_line = b"Header will be skipped\r\ni = [1,2,3]\nj = [1,2,3]\n"
+        expected_result = b"Header will be skipped\r\ni = [1, 2, 3]\nj = [1, 2, 3]\n"
+        response = await self.client.post("/", data=invalid_first_line)
+        self.assertEqual(response.status, 400)
+        response = await self.client.post(
+            "/",
+            data=invalid_first_line,
+            headers={blackd.SKIP_SOURCE_FIRST_LINE: "true"},
+        )
+        self.assertEqual(response.status, 200)
+        self.assertEqual(await response.read(), expected_result)
+
     @unittest_run_loop
     async def test_blackd_preview(self) -> None:
         response = await self.client.post(