Return Python string from dump_as_json() of RF

[hcho3]

The C++ string return value from dump_rf_as_json() is read as bytes in the Python layer. Add decode('utf-8') to obtain a Python string instead.

[GPUtester]

Please update the changelog in order to start CI tests.

View the gpuCI docs here.

[dantegd]

change lgtm

[codecov-io]

Codecov Report

Merging #3130 (f34b64b) into branch-0.17 (25b29f5) will not change coverage.
The diff coverage is 50.00%.

@@             Coverage Diff              @@
##           branch-0.17    #3130   +/-   ##
============================================
  Coverage        69.38%   69.38%           
============================================
  Files              193      193           
  Lines            14704    14704           
============================================
  Hits             10203    10203           
  Misses            4501     4501           

Impacted Files	Coverage Δ
python/cuml/ensemble/randomforestclassifier.pyx	72.72% <50.00%> (ø)
python/cuml/ensemble/randomforestregressor.pyx	70.08% <50.00%> (ø)

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