Don't pass non-array-or-object roots to JSON.generate()

[pda]

If json_pure v2.x is loaded, JSON.generate and JSON.dump reject invalid (non-array, non-object) root level objects. This PR works around this without breaking existing signatures by wrapping invalid root values in an array and then stripping the resulting brackets before input to the HMAC function. It only changes how the HMAC input is generated; it does not change how the final data is encoded.

Previously, without json_pure:

$ docker run ruby:2.3.1 \
    bash -c "gem install signed_json && ruby -e 'require \"signed_json\"; puts SignedJson::Signer.new(\"secret\").encode(nil)'"
Successfully installed signed_json-2.0.0
1 gem installed
["546b281dfcf7e69a4dbcb6a5001929585d65c7d7",null]

Previously, with json_pure v2.0.1:

$ docker run ruby:2.3.1 \
    bash -c "gem install json_pure signed_json && ruby -r json/pure -r signed_json -e 'puts SignedJson::Signer.new(\"secret\").encode(nil)'"
Successfully installed json_pure-2.0.1
Successfully installed signed_json-2.0.0
2 gems installed
/usr/local/bundle/gems/json_pure-2.0.1/lib/json/common.rb:224:in `generate': only generation of JSON objects or arrays allowed (JSON::GeneratorError)
        from /usr/local/bundle/gems/json_pure-2.0.1/lib/json/common.rb:224:in `generate'
        from /usr/local/bundle/gems/json_pure-2.0.1/lib/json/common.rb:394:in `dump'
        from /usr/local/bundle/gems/signed_json-2.0.0/lib/signed_json.rb:50:in `json_generate'
        from /usr/local/bundle/gems/signed_json-2.0.0/lib/signed_json.rb:29:in `digest_for'
        from /usr/local/bundle/gems/signed_json-2.0.0/lib/signed_json.rb:13:in `encode'
        from -e:1:in `<main>'