Switch returned data to use symbols

[blowmage]

This changes the hash key values in the rows of data returned from BigQuery. This change brings BigQuery in line with Spanner.

Before the hash keys were strings:

require "google/cloud/bigquery"

bigquery = Google::Cloud::Bigquery.new
db = bigquery.dataset "mine"

db.query "SELECT * FROM stuff"
rows == [{"name"=>"mike", "foo"=>[{"bar"=>"hey", "baz"=>{"bif"=>1}}, {"bar"=>"world", "baz"=>{"bif"=>2}}]}] #=> true

Now the hash keys are symbols:

require "google/cloud/bigquery"

bigquery = Google::Cloud::Bigquery.new
db = bigquery.dataset "mine"

db.query "SELECT * FROM stuff"
rows == [{ name: "mike", foo: [{ bar: "hey", baz: { bif: 1 } }, { bar: "world", baz: { bif: 2 } }] }] #=> true