Clarify behaviour of client_id with unsigned browser requests

[jogu]

The browser API appendix currently says:

The client_id parameter MUST be omitted in unsigned requests defined in

I think it'd be helpful to specify the behaviour the wallet needs to have. The obvious options are that the wallet should either return an error, or it should ignore the client_id. Specifying the behaviour would allow it to be tested for in the conformance tests.

I'd suggest saying something like:

Wallets MUST ignore client_id if it is included.