REFACTOR Purchasable - getIsAvailable - allow extension

[jsirish]

overhaul tests

[codecov]

Codecov Report

Merging #111 into master will increase coverage by 0.15%.
The diff coverage is 63.63%.

@@             Coverage Diff              @@
##             master     #111      +/-   ##
============================================
+ Coverage     53.00%   53.15%   +0.15%     
- Complexity      238      241       +3     
============================================
  Files            13       13              
  Lines           883      888       +5     
============================================
+ Hits            468      472       +4     
- Misses          415      416       +1     

Flag	Coverage Δ	Complexity Δ
#php	53.15% <63.63%> (+0.15%)	241.00 <0.00> (+3.00)

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Impacted Files	Coverage Δ	Complexity Δ
src/Extension/Purchasable.php	87.28% <63.63%> (-0.33%)	26.00 <0.00> (+3.00)	⬇️

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