react-jss: Observables issue

__Expected behavior:__
Observables should not change the {link: true} to {link: false}

__Describe the bug:__
If Observable's internal method is enumerable, then react-jss considers it a dynamic style and adds an unnecessary empty class. On the other hand, if the method is not enumerable, then the empty class is not added, but it does not link the styles and observable styles do not work.

__Codesandbox link:__
https://codesandbox.io/s/react-jss-v1000alpha-25-observable-issue-g8998

__Versions (please complete the following information):__
- jss: 10.0.0-alpha.25
- Browser [e.g. chrome, safari]: Chrome
- OS [e.g. Windows, macOS]: Windows

Start by reproducing the issue in the linked CodeSandbox with JSS 10.0.0-alpha.25 in Chrome. Trace how react-jss handles observable styles and the link option; done means observable styles work, link remains true, and no unnecessary empty class is added.

Written by the indexing model from the issue text.