Fixes #1246 Missing labels in classification_report

[admo1]

This PR fixes #1246 which reported a bug that caused a problem to occur when users are trying to plot a classifcation_report with labels that were present during training but are missing during scoring.

I have made the following changes:

1. Added labels=self.classes_ argument to sklearn's precision_recall_fscore_support function that is called in the score function of the ClassificationReport class.

CHECKLIST

• [x] Is the commit message formatted correctly?

• [ ] Have you noted the new functionality/bugfix in the release notes of the next release?

• [x] Have you added/updated unit tests where appropriate?

• [x] Have you run the unit tests using pytest?

• [x] Is your code style correct (are you using PEP8, pyflakes)?

[codecov[bot]]

Codecov Report

Merging #1247 (968f060) into develop (d2bafd7) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1247   +/-   ##
========================================
  Coverage    90.61%   90.62%           
========================================
  Files           92       92           
  Lines         5213     5214    +1     
========================================
+ Hits          4724     4725    +1     
  Misses         489      489           

Impacted Files	Coverage Δ
yellowbrick/classifier/classification_report.py	95.65% <100.00%> (+0.04%)	:arrow_up:

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