Fix #378

[ArlindKadra]

Fix ascii decoding error with python 2.

[mfeurer]

Please also write a unit test following the example given in #378.

[codecov-io]

Codecov Report

Merging #418 into develop will decrease coverage by 0.14%.
The diff coverage is 68.75%.

@@ Coverage Diff @@## develop #418 +/- ##
===========================================
- Coverage 89.1% 88.95% -0.15% 
===========================================
Files 32 32 Lines 2671 2681 +10 ===========================================
+ Hits 2380 2385 +5 - Misses 291 296 +5

Impacted Files	Coverage Δ
openml/tasks/split.py	91.08% <66.66%> (-2.67%)	⬇️
openml/datasets/dataset.py	77.64% <66.66%> (-0.22%)	⬇️
openml/datasets/data_feature.py	69.56% <75%> (-0.44%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f4387d6...7ef82d8. Read the comment docs.