When the value of gridSize option is 0, modified to be grouped by the…

[sttkhr]

I want to group by the same coordinates only.
When the value of gridSize option is 0 ,does not work well is converted to 60 .

I modified to be grouped by the same coordinates only ,
When the value of gridSize option is 0.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please let us know the company's name.

[sttkhr]

Sorry.　Right now, I registered CLA.

[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for the commit author(s). If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.

[Remo]

This fix has been applied to our fork https://github.com/gmaps-marker-clusterer/gmaps-marker-clusterer
Thanks for your help