Fix: Always decode JSON to object

[localheinz]

This pull request

• asserts that JSON is consistently decoded to an object
• consistently uses false as argument for the $associative parameter when json_decode()ing a JSON string

[codecov]

Codecov Report

Merging #3 (8be5d6f) into main (b3e8d07) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main       #3   +/-   ##
=========================================
  Coverage     96.66%   96.66%           
  Complexity       14       14           
=========================================
  Files             5        5           
  Lines            30       30           
=========================================
  Hits             29       29           
  Misses            1        1           

Impacted Files	Coverage Δ
src/Json.php	95.45% <ø> (ø)

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