Allow extra field when inserting with prepared queries

Let's say you have a message:

```
class Address(object):

    def __init__(self, street, zipcode, **kwargs):
        self.street = street
        self.zipcode = zipcode

cluster.register_user_type('mykeyspace', 'address', Address)
```

And let's say the type actually contains another field, let's call i `raw_address`

Then inserting data through a prepared statement will actually fail : the driver will complain `raw_address` is missing.

This change addresses that, as any field should be optional.

Author: Mokto

cassandra/cqltypes.py

@@ -1026,7 +1026,7 @@ def serialize_safe(cls, val, protocol_version):
             try:
                 item = val[i]
             except TypeError:
-                item = getattr(val, fieldname)
+                item = getattr(val, fieldname, None)
 
             if item is not None:
                 packed_item = subtype.to_binary(item, proto_version)