fix: fieldvalue gettimestamp

[pmakani]

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Fixes #16

[codecov]

Codecov Report

Merging #279 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #279   +/-   ##
=========================================
  Coverage     77.53%   77.54%           
  Complexity     1151     1151           
=========================================
  Files            75       75           
  Lines          6089     6091    +2     
  Branches        654      654           
=========================================
+ Hits           4721     4723    +2     
  Misses         1019     1019           
  Partials        349      349           

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/com/google/cloud/bigquery/FieldValue.java	82.75% <100.00%> (+0.61%)	24.00 <1.00> (ø)

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