fix: use indexOf to check coverage map

[djedje72]

The "in" operator only check if property exists.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/in

// Arrays
var trees = ['redwood', 'bay', 'cedar', 'oak', 'maple'];
0 in trees        // returns true
3 in trees        // returns true
6 in trees        // returns false
'bay' in trees    // returns false (you must specify the index number, not the value at that index)
'length' in trees // returns true (length is an Array property)
Symbol.iterator in trees // returns true (arrays are iterable, works only in ES2015+)

Current condition is always true.

[codecov-io]

Codecov Report

Merging #60 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #60   +/-   ##
=======================================
  Coverage   97.61%   97.61%           
=======================================
  Files           2        2           
  Lines         126      126           
=======================================
  Hits          123      123           
  Misses          3        3

Impacted Files	Coverage Δ
src/reporter.js	97.7% <100%> (ø)	⬆️

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