Function as does not support quarter parameter

[alfaslash]

Description of the Issue and Steps to Reproduce:

Duration#as('x') method throws an error when trying to pass quarters as an argument, although in the docs it's stated that it should work with all the shorthand keys from moment#add

Link to docs: https://momentjs.com/docs/#/durations/as/

Example:

moment.duration(2, 'd').as('Q');

Result (error):

Uncaught Error: Unknown unit quarter
    at Duration.as (global.js:4193)
    at <anonymous>:1:25

Environment:
All platforms and all browsers.

---

If you are reporting an issue, please run the following code in the environment you are using and include the output:

console.log( (new Date()).toString())   --> "Wed Dec 05 2018 20:46:22 GMT+0500 (Екатеринбург, стандартное время)"
console.log((new Date()).toLocaleString())   --> "05.12.2018, 20:47:24"
console.log( (new Date()).getTimezoneOffset())   --> -300
console.log( navigator.userAgent)   --> "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/70.0.3538.110 Safari/537.36"
console.log(moment.version)   --> "2.22.2"

[ichernev]

Close in favor of #4887