Support for additional path separator

- removed additional argument, set additional separator automatically
instead
- adapted tests
diff --git a/.gitignore b/.gitignore
index 2c3816f..3262812 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,3 +5,6 @@
 .settings
 .project
 .pydevproject
+
+# PyCharm
+.idea/
diff --git a/fake_filesystem_test.py b/fake_filesystem_test.py
index b71553a..9787837 100755
--- a/fake_filesystem_test.py
+++ b/fake_filesystem_test.py
@@ -3006,7 +3006,18 @@
 
 class AdditionalPathSeparatorTest(TestCase):
   def setUp(self):
-    self.filesystem = fake_filesystem.FakeFilesystem(path_separator='!', alt_path_separator='?')
+    self.filesystem = fake_filesystem.FakeFilesystem(path_separator='!')
+    self.filesystem.additional_path_separator = '?'
+
+  def testInitialValue(self):
+    filesystem = fake_filesystem.FakeFilesystem()
+    if self.is_windows:
+      self.assertEqual('/', filesystem.additional_path_separator)
+    else:
+      self.assertIsNone(filesystem.additional_path_separator)
+
+    filesystem = fake_filesystem.FakeFilesystem(path_separator='/')
+    self.assertIsNone(filesystem.additional_path_separator)
 
   def testCollapsePathWithMixedSeparators(self):
     self.assertEqual('!foo!bar', self.filesystem.CollapsePath('!foo??bar'))
@@ -3023,4 +3034,4 @@
 
 
 if __name__ == '__main__':
-  main()
+  unittest.main()
diff --git a/pyfakefs/fake_filesystem.py b/pyfakefs/fake_filesystem.py
index 97e1835..600e426 100644
--- a/pyfakefs/fake_filesystem.py
+++ b/pyfakefs/fake_filesystem.py
@@ -357,18 +357,19 @@
 class FakeFilesystem(object):
   """Provides the appearance of a real directory tree for unit testing."""
 
-  def __init__(self, path_separator=os.path.sep, alt_path_separator=None):
+  def __init__(self, path_separator=os.path.sep):
     """init.
 
     Args:
       path_separator:  optional substitute for os.path.sep
-      alt_path_separator:  optional alternative path separator (especially for '/' under Windows)
 
       Example usage to emulate real file systems:
          filesystem = FakeFilesystem(alt_path_separator='/' if _is_windows else None)
     """
     self.path_separator = path_separator
-    self.alt_path_separator = alt_path_separator
+    self.additional_path_separator = None
+    if _is_windows and path_separator == os.sep:
+      self.additional_path_separator = '/'
     self.root = FakeDirectory(self.path_separator)
     self.cwd = self.root.name
     # We can't query the current value without changing it:
@@ -447,9 +448,9 @@
     Returns:
       The normalized path that will be used internally.
     """
-    if self.alt_path_separator is None or not path:
+    if self.additional_path_separator is None or not path:
       return path
-    return path.replace(self.alt_path_separator, self.path_separator)
+    return path.replace(self.additional_path_separator, self.path_separator)
 
   def CollapsePath(self, path):
     """Mimics os.path.normpath using the specified path_separator.