clear method deletes every object in storage

[janetttt]

I'm submitting a ...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[x] Bug report  <!-- Please check the repository for a similar issue or PR before submitting -->
[ ] Support request => <!-- Please check the repository for a similar issue or PR before submitting -->
[ ] Feature request
[ ] Documentation issue or request

Current behavior
Method clear() of LocalStorageCacheService deletes all objects from the browser's local storage.

Expected/desired behavior
Should only delete objects from the browser's local storage, which are also in the CACHE.

Minimal reproduction of the problem with instructions
Not needed.

What is the motivation / use case for changing the behavior?
I want to be able to delete all object in the cache at once, instead of removing them one by one using the method remove(key: string | number, wild = false).

Environment

• Angular version: 5.x

• Browser:
  N/A

• For Tooling issues:
  N/A

[fulls1z3]

@janetttt sorry for the delay, had busy times. feel free to fire a PR and i'll approve.