proposal: make binding/operation/(input|output)/@messageLabel optional

I know we discussed this before but the way things are is not
consistent. If there's exactly one input or output message in a
MEP there's no need to have the @messageLabel in binding either.

Can we please re-consider this? Look at the syntax summary
section and you'll see what I mean:

http://dev.w3.org/cvsweb/~checkout~/2002/ws/desc/wsdl20/wsdl20.html#Syntax-S
ummary

I'd like to make the attribute optional and keep the same rules
as with interface/operation/(input|output)/@messageLabel.

Thanks,

Sanjiva.