Test output newlines when skipping first line

When skipping the first line of source code, the reference newline must
be taken from the second line of the file instead of the first one, in
case that the file mixes more than one kind of newline character

Signed-off-by: Antonio Ossa Guerra <aaossa@uc.cl>

tests/test_black.py

@@ -355,6 +355,16 @@ def test_skip_source_first_line(self) -> None:
             actual = f.read()
         self.assertFormatEqual(source, actual)
 
+    def test_skip_source_first_line_when_mixing_newlines(self) -> None:
+        code_mixing_newlines = b"Header will be skipped\r\ni = [1,2,3]\nj = [1,2,3]\n"
+        expected = b"Header will be skipped\r\ni = [1, 2, 3]\nj = [1, 2, 3]\n"
+        with TemporaryDirectory() as workspace:
+            test_file = Path(workspace) / "skip_header.py"
+            test_file.write_bytes(code_mixing_newlines)
+            mode = replace(DEFAULT_MODE, skip_source_first_line=True)
+            ff(test_file, mode=mode, write_back=black.WriteBack.YES)
+            self.assertEqual(test_file.read_bytes(), expected)
+
     def test_skip_magic_trailing_comma(self) -> None:
         source, _ = read_data("simple_cases", "expression")
         expected, _ = read_data(