Fixing bug while creating index name

[rralcala]

The wrong variable was used to generate the index name, fixed and added a test to prevent it in the future.
This should fix #39

[codecov-io]

Codecov Report

Merging #40 into master will increase coverage by 0.34%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #40      +/-   ##
=========================================
+ Coverage   51.95%   52.3%   +0.34%     
=========================================
  Files          24      24              
  Lines        1434    1434              
=========================================
+ Hits          745     750       +5     
+ Misses        625     620       -5     
  Partials       64      64

Impacted Files	Coverage Δ
common/elasticsearch/elasticsearch.go	49.61% <100%> (+3.81%)	⬆️

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