fix: get recordvalue by field name

[pmakani]

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Fixes #207

[codecov]

Codecov Report

Merging #718 into master will increase coverage by 0.00%.
The diff coverage is 83.33%.

@@            Coverage Diff            @@
##             master     #718   +/-   ##
=========================================
  Coverage     81.06%   81.07%           
  Complexity     1236     1236           
=========================================
  Files            78       78           
  Lines          6332     6334    +2     
  Branches        703      705    +2     
=========================================
+ Hits           5133     5135    +2     
  Misses          833      833           
  Partials        366      366           

Impacted Files	Coverage Δ	Complexity Δ
...n/java/com/google/cloud/bigquery/BigQueryImpl.java	82.90% <83.33%> (+0.05%)	77.00 <0.00> (ø)

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