Ensure writer -> reader -> writer r/w lock acquisition

Fixes issue #4

fasteners/lock.py

@@ -216,10 +216,11 @@ class ReaderWriterLock(object):
         a lock.
         """
         me = self._current_thread()
-        if self.is_reader():
+        i_am_writer = self.is_writer(check_pending=False)
+        if self.is_reader() and not i_am_writer:
             raise RuntimeError("Reader %s to writer privilege"
                                " escalation not allowed" % me)
-        if self.is_writer(check_pending=False):
+        if i_am_writer:
             # Already the writer; this allows for basic reentrancy.
             yield self
         else:

fasteners/tests/test_lock.py

@@ -315,6 +315,15 @@ class ReadWriteLockTest(test.TestCase):
         self.assertEqual(5, len(writers))
         self.assertEqual(10, len(readers))
 
+    def test_writer_reader_writer(self):
+        lock = fasteners.ReaderWriterLock()
+        with lock.write_lock():
+            self.assertTrue(lock.is_writer())
+            with lock.read_lock():
+                self.assertTrue(lock.is_reader())
+                with lock.write_lock():
+                    self.assertTrue(lock.is_writer())
+
     def test_single_reader_writer(self):
         results = []
         lock = fasteners.ReaderWriterLock()