adding type check for model_type

[angela97lin]

Addresses #85 by checking model_type before processing.

[codecov]

Codecov Report

Merging #109 into master will increase coverage by 0.12%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #109      +/-   ##
==========================================
+ Coverage   92.54%   92.67%   +0.12%     
==========================================
  Files          47       47              
  Lines        1248     1256       +8     
==========================================
+ Hits         1155     1164       +9     
+ Misses         93       92       -1

Impacted Files	Coverage Δ
evalml/tests/automl_tests/test_autoclassifier.py	100% <100%> (ø)	⬆️
evalml/pipelines/utils.py	100% <100%> (+2.85%)	⬆️
evalml/tests/pipeline_tests/test_pipelines.py	93.61% <100%> (+0.28%)	⬆️

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[rwedge]

Looks good!