Bugfix for Cybersource. Wrong exception was except

Bugfix and added documentation about a feature
Optional parameters
payment.attrs.fingerprint_session_id
payment.attrs.merchant_defined_data
were essentially required otherwise an error was thrown.
Was caused by the wrong exception being captured

docs/backends.rst

@@ -110,6 +110,12 @@ accomplished by passing your data to the :class:`Payment` instance::
 
       >>> payment.attrs.merchant_defined_data = {'01': 'foo', '02': 'bar'}
 
+Fingerprinting::
+
+Cybersource allows you to pass a fingerprint data to help identify fraud
+
+      >>> payment.attrs.fingerprint_session_id
+
 
 Dotpay
 ------

payments/cybersource/__init__.py

@@ -280,7 +280,7 @@ def _get_params_for_new_payment(self, payment):
         }
         try:
             fingerprint_id = payment.attrs.fingerprint_session_id
-        except KeyError:
+        except AttributeError:
             pass
         else:
             params["deviceFingerprintID"] = fingerprint_id
@@ -446,7 +446,7 @@ def _prepare_items(self, payment):
     def _prepare_merchant_defined_data(self, payment):
         try:
             merchant_defined_data = payment.attrs.merchant_defined_data
-        except KeyError:
+        except AttributeError:
             return
         else:
             data = self.client.factory.create("data:MerchantDefinedData")