Serialise dates to UTC

[via-guy]

If a DateTime object is serialised for an HTTP request but it's not in UTC then the ISO8601 string will be invalid, as it won't contain the timezone (missing from the dart implementation of DateTime). This causes requests to either fail or use the server timezone, which may not be the same as that of the device.

I know that I better solution would be to fix the DateTime function toIso8601String, but this is an easy solution here.

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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).

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[via-guy]

@googlebot I signed it!

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[guykogus]

@googlebot I consent.

[kevmoo]

I'm hesitant to take this PR, just because it changes the behavior of something that's been VERY stable for a long time.

I think I'd rather consider using toIso8601String in a breaking change...

Want to open an issue for that?