format callback context properly

[eduardomourar]

Issue #, if available: #15

Description of changes:

This is will properly format the callback context even from different types (array, object literal or Map) that were serialized.

[codecov]

Codecov Report

Merging #16 into master will decrease coverage by 0.04%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master      #16      +/-   ##
==========================================
- Coverage   94.49%   94.45%   -0.05%     
==========================================
  Files          11       11              
  Lines         781      793      +12     
  Branches      112      118       +6     
==========================================
+ Hits          738      749      +11     
- Misses         43       44       +1     

Flag	Coverage Δ
#unittests	94.45% <93.75%> (-0.05%)	⬇️

Impacted Files	Coverage Δ
src/utils.ts	83.05% <0.00%> (ø)
src/resource.ts	94.64% <100.00%> (+0.41%)	⬆️
src/proxy.ts	80.35% <0.00%> (-1.79%)	⬇️

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