Add a :sort_keys option to Encoder

[NZKoz]

This allows people to canonicalize JSON and ensure documents are identical irrespective of the order
keys were added to the underlying Hash. This was covered previously in #7 but in the intervening time Symbol#<=> has shipped so I figured I'd see how hard it was to hack in.

This is mostly provided as a "sounding board" to see if the feature is useful to anyone else, rather than something actually ready for inclusion.. The sorting is pretty hackish as is, it should probably intern then sort rather than sort then intern etc.

[brianmario]

Yeah I can dig this. I just noticed the golang JSON encoder does this too, probably for similar reasons (consistent output). And the implementation is fine by me 👍

[NZKoz]

The tests only really work on 1.9 and above, the 1.8 hash ordering is likely to break things but I'm 2.0 everywhere so don't really ... care... It's been so long that I'm not sure if @TobiX had any thoughts on additional test cases to include?

[d]

@NZKoz I think the key sorting will work for 1.8 as well coz Hash#keys gives an array, and Array#sort is non-destructive. The tests are only failing because of activesupport which is a benchmark-only dependency.

Jesse

[NZKoz]

@d the sort will break if they keys contain things other than strings, because the sort happens before the to_s, gets called on them, this is obviously pretty minor and would take some crazy test cases, but I'm sure someone could break it.

[NZKoz]

Yeah, so there's a test case that fails with the current non-refactored implementation of keys / sorting.