Define `HEADER_FIELD_ENCODE_SET`

[Ralith]

This is needed for encoding UTF-8 in header fields, e.g. Content-Disposition: attachment; filename*=UTF-8''.... Defined in RFC8187. I'd appreciate a double-check that I got the characters right.

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This change is 

[SimonSapin]

The define_encode_set! macro is intended to be usable in other crates. Is there a reason this specific set should be defined in this crate?

[Ralith]

Because it's a standardized encoding commonly needed by both web services and clients, similar to the others. Correct implementations of the Content-Disposition header are a wonderful thing.

[SimonSapin]

Still, wouldn’t this belong more in a crate like http? Even in things with RFCs there are many that use precent-encoding slightly differently, I’d prefer this crate not to be responsible for them all.

[Ralith]

The http crate does not depend on this crate. Is that likely to change?

[SimonSapin]

Closing as out of scope. Even with this PR, this crate does not provide an implementation (correct or not) of the Content-Disposition header.