Add support to encode multiple scopes on all grant types

[jonathansamines]

• Add support to encode multiple scopes on all grant types
• Add support to to override the grant_type parameter name
• Don't mutate params provided to grants token requests

[koistya]

Setting a custom grant_type doesn't seem to work:

Error: The content-type is not JSON compatible
    at finish (.yarn/cache/@hapi-wreck-npm-17.1.0-bed654125d-ea3fd37926.zip/node_modules/@hapi/wreck/lib/index.js:383:42)
    at wrapped (.yarn/cache/@hapi-hoek-npm-9.1.1-9cb487d073-e5be371c57.zip/node_modules/@hapi/hoek/lib/once.js:17:13)
    at module.exports.internals.Recorder.onReaderFinish (.yarn/cache/@hapi-wreck-npm-17.1.0-bed654125d-ea3fd37926.zip/node_modules/@hapi/wreck/lib/index.js:436:20)
    at Object.onceWrapper (events.js:421:28)
    at module.exports.internals.Recorder.emit (events.js:327:22)
    at finish (internal/streams/writable.js:657:10)
    at processTicksAndRejections (internal/process/task_queues.js:80:21)

[jonathansamines]

Hi @koistya Please open a new issue with an isolated reproduction example of your problem.