fix source_to_unicode py3 compatibility

test/test_regression.py

@@ -14,6 +14,7 @@ import jedi
 from jedi._compatibility import u
 from jedi import Script
 from jedi import api
+from jedi import common
 from jedi.evaluate import imports
 from jedi.parser import Parser, load_grammar
 
@@ -156,6 +157,15 @@ class TestRegression(TestCase):
             "abc()."
         assert Script(s).completions()
 
+    def test_source_to_unicode_unicode_text(self):
+        source = (
+            b"# vim: fileencoding=utf-8\n"
+            b"# \xe3\x81\x82\xe3\x81\x84\xe3\x81\x86\xe3\x81\x88\xe3\x81\x8a\n"
+        )
+        actual = common.source_to_unicode(source)
+        expected = source.decode('utf-8')
+        assert actual == expected
+
 
 def test_loading_unicode_files_with_bad_global_charset(monkeypatch, tmpdir):
     dirname = str(tmpdir.mkdir('jedi-test'))