Update extendObservable for TS 2.6
Update extendObservable for TS 2.6

In Typescript 2.6, Object.getOwnPropertyDescriptor now returns
`PropertyDescriptor | undefined`. Assert that the type is still just
`PropertyDescriptor`, because it is checked with
`hasOwnProperty` just above.

mweststrate/MOBservable,mobxjs/mobx,mobxjs/mobx,mobxjs/mobx,mobxjs/mobx,mweststrate/MOBservable,mweststrate/MOBservable