Issue title: Compiling autogenerated code with -Wextra
 Issue body: I prefer to compile me code with the -Wextra option (among others) warnings and it is almost possible with the autogenerated stubs from the jsonrpcstub program.

The only issue is, that if an autogenerated method doesn't accept any parameters, GCC (and possibly others) will warn that "request" is an unused parameter.

I find the "unused parameter" warning fairly useful, so I would prefer to keep that warning.

I have worked around it by adding `this->cg.writeLine("(void)request;");` at the start of the function body generation in cppserverstubgenerator.cpp, but I'm not sure this is the nicest solution.

I could also hack around my build scripts to build the stubgenerator with less warnings enabled, but that would be a bit of a shame.

So in short, it seems like the code can _almost_ compile with -Wextra and I personally find it worth the minor effort to make it able to compile cleanly with -Wextra.

Thanks.

 Comments: 
 Comment 0: You are absolutely right. It should compile without warnings. I will fix that. Thanks for the hint.

 Comment 1: Fixed in: 9c67561fac30e6d17f7a11583cc6140892b04545

 Comment 2: Perfect!

In non-autogenerated code I prefer to just not give the parameter a name which will also suppress the warning, but in this case I think the solution is just fine.

Also good job in keeping the tests up to date and useful.

I don't mean to sound like your boss giving you code reviews or something, I just think you deserve some credit for a good job. :-)

Once again thanks for the quick reply and fix.

 Comment 3: Thanks. Code reviews are one of the best things that can happen to an open source project :+1: 

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