Changes for modified Firebase's JWT structure.

[amolchakane]

Firebase has modified their JWT structure. Considering their documentation these changes are implemented and tested.

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[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).

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[amolchakane]

I signed it!

[googlebot]

CLAs look good, thanks!

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CLAs look good, thanks!

[douggr]

PHP parse error.

[amolchakane]

Mistakenly added letter 'c'. Removed it now.