Change resolved Accept-Encoding to honour server side order preferece

[gmokki]

...when comparing client provided encodings with equal quality
Also change the quality of automatically added identity encoding to be lower than other encodings instead of equal to lowest seen quality.

This change is required by pillarjs/send#108 to be able to use the negotiator.

[dougwilson]

Thank you, @gmokki! Sorry I have been away for a while :( Looking at the change, it doesn't seem like a backwards-compatible change, does it? Mainly, I'm concerned because Express 4.x directly exposes the results of this module, and just do not have the bandwidth to maintain two version lines.

1. Would it be possible to place the behavior change behind an opt-in flag?
2. This seems like a change that should uniformly be applied across all the different negotiations. Can that be added to maintain a uniform API?

[gmokki]

You are correct that the change is not backwards compatible. I can put the different sorting order behind an option.

However, I'd like to argue that because current top browsers:

• put "gzip" as the first accept-encoding
• do not use quality levels

Then any user of negotiator.accept(['anything', 'gzip', 'other']) will have always selected gzip. And basically all the other options have been always been ignored.
Because the selection has never worked in real life I hope the accept with array argument has never actually been used in real projects and thus breaking backwards compatibility should not break anything badly.

But again, I'll try to add a commit with configurable default so that you can choose take it if you prefer avoiding backwards incompatible changes.

[dougwilson]

It doesn't matter what browsers do, as there is no reason why this module would not be used in REST APIs getting requests from non-browser clients.

It is a breaking change, regardless of speculation.

[federomero]

Hi, I just want to point out that I think this change breaks compliance with RFC 2616.

I couldn't find anywhere in the RFC that the order of the encodings indicates any kind of preference from the client. But it's explicitly stated in the RFC that:

If more than one media range applies to a given type, the most specific reference has precedence

Even though that sentence refers to the Accept header, I think the same principle applies to Accept-Encoding.

I don't mind honouring client order, but I wouldn't do it at the cost honouring specificity.

[gmokki]

I have now rewritten the patch so that the sorting algorithm can be chosen by passing suitable options object. The current algorithm is maintained as the default.

Do I need to add similar functionality also to the other parsers or do you think adding options only to encoding parser is acceptable?