fix: fix conversion for pre-epoch timestamps

[chingor13]

Fixes #159

[codecov]

Codecov Report

Merging #160 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #160      +/-   ##
============================================
+ Coverage     66.54%   66.56%   +0.01%     
- Complexity      370      371       +1     
============================================
  Files            34       34              
  Lines          1877     1881       +4     
  Branches        240      241       +1     
============================================
+ Hits           1249     1252       +3     
- Misses          526      527       +1     
  Partials        102      102

Impacted Files	Coverage Δ	Complexity Δ
...core/src/main/java/com/google/cloud/Timestamp.java	90.62% <100%> (+0.62%)	25 <2> (+1)	⬆️
.../java/com/google/cloud/testing/CommandWrapper.java	87.87% <0%> (-9.1%)	13% <0%> (ø)
...src/main/java/com/google/cloud/ServiceOptions.java	44.26% <0%> (+0.79%)	33% <0%> (ø)	⬇️

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[elharo]

also, haven't we seen this problem before a few months back? maybe in a different project?

[chingor13]

also, haven't we seen this problem before a few months back? maybe in a different project?

I think this was something similar: googleapis/google-http-java-client#895