fix: ordering keys publishing of last batch

[kamalaboulhosn]

Ensure that the last batch for an ordering key gets published.

[codecov]

Codecov Report

Merging #9 into master will increase coverage by 0.04%.
The diff coverage is 81.81%.

@@             Coverage Diff             @@
##             master      #9      +/-   ##
===========================================
+ Coverage     78.76%   78.8%   +0.04%     
- Complexity      285     288       +3     
===========================================
  Files            21      21              
  Lines          2566    2576      +10     
  Branches        125     128       +3     
===========================================
+ Hits           2021    2030       +9     
  Misses          490     490              
- Partials         55      56       +1

Impacted Files	Coverage Δ	Complexity Δ
...gle/cloud/pubsub/v1/SequentialExecutorService.java	90.8% <100%> (ø)	0 <0> (ø)	⬇️
...ain/java/com/google/cloud/pubsub/v1/Publisher.java	89.07% <80%> (+0.03%)	43 <2> (+3)	⬆️
...ava/com/google/cloud/pubsub/v1/OpenCensusUtil.java	71.66% <0%> (ø)	5% <0%> (ø)	⬇️

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