Decorate requests with symbols with apply=true

[alexghr]

Hello,

This PR fixes a bug where Symbol decorations would not get added to request instances when { apply: true } option was used. The issue was caused by iterating over decorations object using for...in which only iterates over enumerable properties (symbols are not enumerable)

Since Symbols are expected property names this PR fixes the issue above by also iterating over known symbol properties using Object.getOwnPropertySymbols.

[kanongil]

I would probably refactor requestApply, along with the other _decorations entries, to use a Map(). This makes it explicit which properties are decorated, and should fix this bug.

[alexghr]

Thanks for the suggestion @kanongil. It simplifies the code greatly! I'm not very familiar with the codebase but I've tried my best at refactoring the internal _decorations object to use a series of maps.

The test still pass apart from two of them, although they were failing even before this change.

[hueniverse]

It's rare I merge a PR... nicely done.