DispatchError changes after encoding and then decoding

[shaunxw]

If we have a DispatchError::Module variant like this:

DispatchError::Module { index: 1, error: 1, message: Some("foo") }

After encoding and then decoding, it will become different:

DispatchError::Module { index: 1, error: 1, message: None }

IMHO it needs be fixed as in general variables should be the same. It could cause troubles while doing equal assertion. Like in ORML unit tests, when a event contains DispatchError, comparing the built even in test function with System::events() results unexpected unequal, unless with a manual conversion like here.

[bkchr]

This is done by intention, because we don't want to store the message on chain.

I'm open to accept a pr that changes the PartialEq to ignore message.