Fixes the issue of stringify encoding comma(used to join array) even when `encodeValuesOnly = true`

[adnan-creator]

Following changes were made:

• According to the documentation when encodeValuesOnly = true, then all characters except the values must not be encoded. This includes brackets and joining characters such as comma. It works perfectly for all other characters except when comma is used to join elements of an array. Then even the comma is encoded.
• One of the tests had a bug where we assumed the result of qs.stringify({ a: ['b', 'c', 'd'] }, { arrayFormat: 'comma' }) to be equal to 'a=b,c,d'. Which is not the expected behavior as for the comma to not be encoded we must either set encode: false or encodeValuesOnly: true. So the right result would be 'a=b%2Cc%2Cd' as the default functionality of the function is to encode the complete string.

[adnan-creator]

Thanks! for now these tests seem fine.

[codecov[bot]]

Codecov Report

Merging #424 (5dbeeb4) into master (e2fd364) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #424   +/-   ##
=======================================
  Coverage   99.78%   99.78%           
=======================================
  Files           8        8           
  Lines        1388     1393    +5     
  Branches      167      169    +2     
=======================================
+ Hits         1385     1390    +5     
  Misses          3        3           

Impacted Files	Coverage Δ
lib/stringify.js	99.29% <100.00%> (+0.03%)	:arrow_up:
test/stringify.js	99.75% <100.00%> (-0.01%)	:arrow_down:

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