[MRG] TST: add test for creating datset from NumPy array

[glemaitre]

Reference Issue

closes#537

What does this PR implement/fix? Explain your changes.

Implement a unit test for the function create_dataset from NumPy array.

[mfeurer]

Looks great! Thanks a lot.

[codecov-io]

Codecov Report

Merging #539 into develop will increase coverage by 0.24%.
The diff coverage is n/a.

@@ Coverage Diff @@## develop #539 +/- ##
===========================================
+ Coverage 89.82% 90.07% +0.24% 
===========================================
Files 32 32 Lines 2811 2811 ===========================================
+ Hits 2525 2532 +7 + Misses 286 279 -7

Impacted Files	Coverage Δ
openml/datasets/dataset.py	78.22% <0%> (+0.36%)	⬆️
openml/datasets/functions.py	90.38% <0%> (+2.88%)	⬆️

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