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In SocketRocket the 'extensions' attribute is missing as well
From what I gather from the RFC, extensions are more something that is dealt with by the client. Currently, there are no extensions that SocketRocket supports, so I don't feel it is necessary to add this yet. Once I get a better understanding of their use (when formal extensions come to be) I will definitely add it.
Why not keeping the issue open, e.g. for future release ?
thx for keeping it :)