fix: selectedFields on getRows not working correctly

[laljikanjareeya]

Fixes #702

Data from Back-end is retrieving correctly, but mapping of the data with the schema fields has an issue.
On below line schema fields and data mapping is based on index so, it will map always first property even if user doesn't selected that property.

nodejs-bigquery/src/bigquery.ts

Line 387 in 93945dd

const schemaField = schema.fields![index];

Solution:
Back-end data is retrieving same order as schema fields order, solution for this is to filter schema based on selected fields when selectedFields is provided by user.

[codecov]

Codecov Report

Merging #712 into master will increase coverage by 0.00%.
The diff coverage is 97.56%.

@@           Coverage Diff           @@
##           master     #712   +/-   ##
=======================================
  Coverage   95.16%   95.17%           
=======================================
  Files           7        7           
  Lines        6311     6341   +30     
  Branches      369      371    +2     
=======================================
+ Hits         6006     6035   +29     
- Misses        305      306    +1     

Impacted Files	Coverage Δ
src/bigquery.ts	99.94% <97.22%> (-0.06%)	⬇️
src/table.ts	99.95% <100.00%> (ø)

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[steffnay]

Looks good to me. Pending second review from @stephenplusplus

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[laljikanjareeya]

@googlebot I fixed it.

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