Added fix to prevent error for Firestore v5.5.0

[lucacasonato]

This message shows up now... added the fix as described.

@firebase/firestore: Firestore (5.5.0): 
The behavior for Date objects stored in Firestore is going to change
AND YOUR APP MAY BREAK.
To hide this warning and ensure your app does not break, you need to add the
following code to your app before calling any other Cloud Firestore methods:

  const firestore = firebase.firestore();
  const settings = {/* your settings... */ timestampsInSnapshots: true};
  firestore.settings(settings);

With this change, timestamps stored in Cloud Firestore will be read back as
Firebase Timestamp objects instead of as system Date objects. So you will also
need to update code expecting a Date to instead expect a Timestamp. For example:

  // Old:
  const date = snapshot.get('created_at');
  // New:
  const timestamp = snapshot.get('created_at');
  const date = timestamp.toDate();

Please audit all existing usages of Date when you enable the new behavior. In a
future release, the behavior will change to the new behavior, so if you do not
follow these steps, YOUR APP MAY BREAK.

[codecov]

Codecov Report

Merging #32 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #32   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           6      6           
  Lines          85     85           
  Branches       18     18           
=====================================
  Hits           85     85

Impacted Files	Coverage Δ
src/FirestoreProvider.js	100% <ø> (ø)	⬆️

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[green-arrow]

@CreativeGuy2013 - Thanks for taking the time to open a PR for this! Since this is opting into potentially breaking functionality, I would rather have the user pass in a prop that explicitly opts into this new behavior.

Perhaps we could add a prop called useTimestampsInSnapshots that defaults to false, and pass that to the firestore initialization.

Would you be open to updating this PR with that and adding a bit in the README in the FirestoreProvider section about the new prop?

Thanks again!

[lucacasonato]

Everything should work using useTimestampsInSnapshots. The default is false.

[green-arrow]

@CreativeGuy2013 - Thanks again for the PR and for the quick updates on this!