Reset DedupeIntegration's last-seen if before_send dropped the event

[sentrivana]

Imagine an app throws an exception twice, from different places. The first exception is dropped in the user's before_send. The second exception is not. Should the second exception appear in Sentry?

The current state is that it won't, since DedupeIntegration will take the first, dropped exception into account. When encountering the second exception, it'll consider it a duplicate and will drop it, even though the first exception never made it to Sentry.

In this PR, we reset DedupeIntegration's last-seen if an event has been dropped by before_send, ensuring that the next exception will be reported.

Closes #371

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 3 lines in your changes missing coverage. Please review.

Project coverage is 79.55%. Comparing base (65132ba) to head (905fff7).
Report is 1 commits behind head on master.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
sentry_sdk/integrations/dedupe.py	66.66%	1 Missing and 1 partial ⚠️
sentry_sdk/client.py	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4142      +/-   ##
==========================================
- Coverage   79.58%   79.55%   -0.03%     
==========================================
  Files         141      141              
  Lines       15723    15732       +9     
  Branches     2673     2675       +2     
==========================================
+ Hits        12513    12516       +3     
- Misses       2367     2370       +3     
- Partials      843      846       +3     

Files with missing lines	Coverage Δ
sentry_sdk/client.py	79.17% <66.66%> (-0.08%)	⬇️
sentry_sdk/integrations/dedupe.py	80.00% <66.66%> (-2.76%)	⬇️

... and 4 files with indirect coverage changes

[antonpirker]

I guess there code be an edge case that causes problem. (see my comment)

[antonpirker]

looks good