Fix: assert.Subset for maps

[Agurato]

Summary

This PR aims to fix the assert.Subset (and assert.NotSubset) functions for maps

Changes

• Updated the NotSubset(…) function to check the key's existence in the containing map. Before the fix, if the key did not exist in the containing map, a panic would occur and the function would return false.

• Updated the Subset(…) function to do the same and avoid a panic. This panic was well intercepted and the test returned the correct result, but I figured that this was a better way.

Motivation

I noticed that the NotSubset function would not work properly for intersecting maps.

For example, I wanted to check that { "a": "x", "d": "z"} is indeed not a subset of { "a": "x", "b": "y", "c": "z"}

Related issues

#1173

[dolmen]

Please rebase your branch on top of master and fix conflicts.

[Agurato]

This has been fixed in #1261