Fixes #1092
Fixes #1092

- When returning the mobx administration for an object checks that the
administration is an "ownProperty" of the object. Otherwise create a
"local" administration
- The 3 tests added to show the bad behavior described by #1092 works
fine

mweststrate/MOBservable,mweststrate/MOBservable,andykog/mobservable,mweststrate/MOBservable,mobxjs/mobx,andykog/mobx,mobxjs/mobx,mobxjs/mobx,mobxjs/mobx,andykog/mobx,andykog/mobservable