fix: Send event timestamp

[rhcarvalho]

Fixes #2573.

[rhcarvalho]

Moved lines around just to sort alphabetically.

[thinkocapo]

@rhcarvalho I like that timestamp is set now.

One thing I noticed, it looks like the js event gets timestamped in format 1590946750.683085

But I'm pretty sure that sentry-python timestamps too but it's in format 2020-05-31T11:10:29.118356Z

Is there one that's preferred? It looks like we support both
https://develop.sentry.dev/sdk/event-payloads/properties/span_timestamp/

Update - I found on Transactions there's a start_timestamp and a timestamp and I've developed program to handle Errors vs Transactions. I realized updating timestamps (while pulling events out of a database for making them real-time again) is easy on Errors because on Transactions I think there's a Start+Stop (start_timestamp, timestamp)

I'm good from here.