Resolve a possible explodeMap column names collision

[pomadchin]

This PR complements #488 and makes sure that no naming collision happens when performing the explodeMap.

[codecov]

Codecov Report

Merging #582 (2d43512) into master (20bb12a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #582   +/-   ##
=======================================
  Coverage   95.14%   95.14%           
=======================================
  Files          65       65           
  Lines        1134     1134           
  Branches        6        5    -1     
=======================================
  Hits         1079     1079           
  Misses         55       55           

Flag	Coverage Δ
2.12.15	95.14% <ø> (ø)
2.13.7	95.78% <ø> (ø)

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Impacted Files	Coverage Δ
...ataset/src/main/scala/frameless/TypedDataset.scala	100.00% <ø> (ø)

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