Fix ms.hgetall return value interpretation

[scottinet]

Description

(this PR should not appear in the changelog: it affects only the changes on the MemoryStorage controller in the 4.x SDK version)

The hgetall result was interpreted as an array of, alternatively, key and its associated value, and a mapper function was plugged into this method to ensure a proper JSON object was returned to the client.
But the result of this redis call is already a POJO, so the mapping is unnecessary and causes a SDK crash.

[codecov-io]

Codecov Report

Merging #196 into 4.x will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##              4.x     #196      +/-   ##
==========================================
+ Coverage   99.36%   99.41%   +0.05%     
==========================================
  Files          16       16              
  Lines        1721     1713       -8     
  Branches      458      457       -1     
==========================================
- Hits         1710     1703       -7     
+ Misses         11       10       -1

Impacted Files	Coverage Δ
src/MemoryStorage.js	99.01% <ø> (-0.08%)	❌
src/Kuzzle.js	99.63% <0%> (+0.18%)	✅

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