Make item_assets a dict

[kylebarron]

Closes #46

[vincentsarago]

@kylebarron can you share an item and add a test ?

[kylebarron]

Test added

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@e4a3f33). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #47   +/-   ##
=========================================
  Coverage          ?   99.04%           
=========================================
  Files             ?       29           
  Lines             ?      629           
  Branches          ?        0           
=========================================
  Hits              ?      623           
  Misses            ?        6           
  Partials          ?        0           

Flag	Coverage Δ
unittests	99.04% <0.00%> (?)

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