Add ability to have custom 405 handler

[meztez]

Fixes #757

I could not explain it better than #757

This is crude, I would still prefer to have a more global approach to custom error handling

PR task list:

• Update NEWS
• Add tests
• Update documentation with devtools::document()

[cpsievert]

set404Handler should maybe do the same (to avoid errors that aren't caught until runtime)?

Suggested change

	private$unallowedMethodHandler <- fun
	stopifnot(is.function(fun))
	args <- names(formals(fun))
	if (!all(c("req", "res") %in% args)) {
	stop("fun must have formal arguments for req and res")
	}
	private$unallowedMethodHandler <- fun

[meztez]

There are a couple other plumber functions that take a function as input and are executed at runtime with potential failure. This might be material for another PR

[schloerke]

@meztez Would you like / have time to do a PR for the other method parameter checks?

[meztez]

Eventually, I still have the regular job thing to do 😄

[meztez]

Scrath this PR, I'm thinking long term we should have a better way to handler errors.

Something like serializers and parsers, a collection of error handler. This PR is more a bandaid than a real solution.