Python: adjusted input.read(size) argument value interpretation.

Previously, passing 0 resulted in reading the whole body and all negative
values raised an exception.

Now the behaviour is in consistentance with io.RawIOBase.read() interface,
and passing 0 returns empty (byte) string, while -1 results in reading the
whole body.

test/test_python_application.py

@@ -243,7 +243,6 @@ Connection: close
 
         self.assertEqual(self.post(body=body)['body'], body, 'input iter')
 
-    @unittest.expectedFailure
     def test_python_application_input_read_length(self):
         self.load('input_read_length')