Fix storage bug

[qlchan24]

The quantity field was not assigned to the updated quantity in the previous jsonAdaptedProduct causing the quantities of product not to be updated through update command

[codecov-io]

Codecov Report

Merging #117 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #117      +/-   ##
============================================
+ Coverage     69.99%   70.01%   +0.01%     
  Complexity      551      551              
============================================
  Files            89       89              
  Lines          1853     1854       +1     
  Branches        217      217              
============================================
+ Hits           1297     1298       +1     
  Misses          508      508              
  Partials         48       48              

Impacted Files	Coverage Δ	Complexity Δ
.../java/seedu/clinic/storage/JsonAdaptedProduct.java	85.18% <100.00%> (+0.56%)	4.00 <0.00> (ø)

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[jeffreytjs]

LGTM