request RequestOptions interface should not contain handleAs

[tomdye]

Line: https://github.com/dojo/core/blob/master/src/request.ts#L127
The RequestOptions object should not contain handleAs as it's functionality is superseded by the responseType flag.

• Any current uses of handleAs: 'json' can be replaced with responseType: 'json'

[kitsonk]

@bryanforbes it would be great to get your opinion if this was intended or not.

[tomdye]

@bryanforbes commented in a chat window previously saying that a decision had been made to go with responseType over handleAs. The design documentation does not include handleAs in the interface for RequestOptions