Handle the slice object case in TransformGenerator.__getitem__
diff --git a/pathod/language/generators.py b/pathod/language/generators.py
index 01f709e2b0..9fff3082f0 100644
--- a/pathod/language/generators.py
+++ b/pathod/language/generators.py
@@ -37,6 +37,8 @@ def __len__(self):
 
     def __getitem__(self, x):
         d = self.gen.__getitem__(x)
+        if isinstance(x, slice):
+            return self.transform(x.start, d)
         return self.transform(x, d)
 
     def __repr__(self):

should fix https://travis-ci.org/mitmproxy/mitmproxy/jobs/134827120#L501 test failures:

```
self = <netlib.websockets.protocol.Masker object at 0x7fe8ebdfc110>
offset = slice(0, 3, None), data = 'foo'
    def mask(self, offset, data):
        result = bytearray(data)
        if six.PY2:
            for i in range(len(data)):
>               result[i] ^= ord(self.key[offset % 4])
E               TypeError: unsupported operand type(s) for %: 'slice' and 'int'
netlib/websockets/protocol.py:47: TypeError
```
