Add the ability to marshal a Response to SGML/XML and test it

This allows for ofxgo to be used to create well-formatted OFX from poor
OFX, or even be used to generate OFX from other formats for easier
importing into financial management software.

Test this functionality by adding "round trip" testing to all existing
tests - ensure that responses' content is the same after a round trip of
marshalling and unmarshalling them.

bank_test.go

@@ -283,4 +283,5 @@ func TestUnmarshalBankStatementResponse(t *testing.T) {
 	}
 
 	checkResponsesEqual(t, &expected, response)
+	checkResponseRoundTrip(t, response)
 }