Support all PUT operations, error later

[RubenVerborgh]

When executing a bodyless PUT request, I would get:

UnsupportedHttpError: No handler supports the given input: [This handler only supports DELETE operations., This handler only supports GET operations., This handler only supports PATCH operations., This handler only supports POST operations., PUT operations require a body.].
    at CompositeAsyncHandler.findHandler (/Users/ruben/Documents/UGent/Solid/solid-community-server/src/util/CompositeAsyncHandler.js:77:15)
    at processTicksAndRejections (internal/process/task_queues.js:97:5)

Clearly, the PUT handler is the (only) right one. The fact that it errors is because the request is invalid, and the PUT handler is the only one who knows that.

So the right error is “no body specified”, all the rest is noise. This PR brings it to:

UnsupportedHttpError: PUT operations require a body.

Apart from fixing this case, I also want to draw attention to the generic mechanism. canHandle answers the question: “is this the right actor to be handling this?’, not: “will handling succeed?”. In this case, the PUT actor can handle; it's just that the correct handling is to error. This means two things in particular:

• canHandle methods should only check for applicability, not avoid any other errors via preconditions
• calls to handleSafe are not guaranteed to not error; only to have that error thrown by the appropriate actor

I have created #119 to ensure this is applied consistently.