Remove `Accept` header setup for each request individually

[maratal]

I've just noticed we should probably also set the Accept header on the request, [again similarly to -[ARTRest time:]]

I think it's a legacy code. Accept header is set later here for all requests: https://github.com/ably/ably-cocoa/blob/0fe3c59658ce7d0b7181571cdad1f068d4085088/Source/ARTRest.m#L313

Originally posted by maratal in https://github.com/ably/ably-cocoa/pull/1420#discussion_r917117828

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