Make ISO8601UtilsTest run successfully in any time zone

[danielthegray]

In some timezones, ISO8601UtilsTest fails with:

Failed tests:   testDateFormatString(com.google.gson.internal.bind.util.ISO8601UtilsTest):
      expected:<2018-06-2[5]> but was:<2018-06-2[4]>

This PR simply forced the timezone to always be UTC so that the test passes regardless of the timezone of the person running the test.

[google-cla]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). 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 (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[danielthegray]

@googlebot I signed it!

[Marcono1234]

Thanks for the pull request, but there are already three existing ones for this issue:

• Specify timezone for GregorianCalender in testDateFormatString() #1665
• Fix ISO8601UtilsTest failing on systems with UTC+X #1687
• ISO8601UtilsTest.testDateFormatString: Create date instance with UTC #1781

[danielthegray]

Ahh apologies... I was looking for something too specific. I hope something gets merged soon!

[danielthegray]

The other pull requests are more complete and solve the problem. It should not be hard to get them merged in! Some action would be nice on the maintainer's side!