Add the utf-8 bom to the prefix in the tokenizer.

test/test_prefix.py

@@ -1,9 +1,12 @@
 from itertools import zip_longest
+from codecs import BOM_UTF8
 
 import pytest
 
 import parso
 
+unicode_bom = BOM_UTF8.decode('utf-8')
+
 
 @pytest.mark.parametrize(('string', 'tokens'), [
     ('', ['']),
@@ -48,6 +51,7 @@ def test_simple_prefix_splitting(string, tokens):
     ('\\\n', ['backslash', 'spacing']),
     (' \t', ['spacing']),
     (' \t ', ['spacing']),
+    (unicode_bom + ' # ', ['bom', 'comment', 'spacing']),
 ])
 def test_prefix_splitting_types(string, types):
     tree = parso.parse(string)