observer-util fails to queue reactions in Edge 14

[lukeburns]

Was doing testing in Edge 14, and found that observer-util doesn't queue reactions: the reason is Edge doesn't implement WeakMap properly chakra-core/ChakraCore#2419 and the set handler returns before queueing at https://github.com/nx-js/observer-util/blob/master/src/handlers.js#L80. Just putting this here in case it's helpful to anyone else targeting older Edge versions.

[solkimicreb]

I should probably setup SauceLabs for the project.