Removing "DEFAULT" value in terminals

[weixuanfu]

1. Fix a bug that "DEFAULT" in the parameter(s) of nested estimator causes KeyError when exporting pipeline, which is related to the issue fit() doesn't end properly (no best pipeline) #499

2. Add a unit test for this issue

The 2 lines in operator_utils.py cause the bug in the issue #499. It works all right with operators without nested/stacked estimator as parameter(s) but it will cause KeyError once dep_op_arguments do not have nested/stacked estimator's name as key of the dictionary. It means that if any parameter is DEFAULT in the nested/stacked estimator would cause the whole pipeline cannot exported as valid scikit-learn Pipeline object.

[coveralls]

Coverage increased (+3.4%) to 91.797% when pulling 3537317 on weixuanfu2016:issue499 into c427093 on rhiever:development.

[weixuanfu]

Update: after discussions with @rhiever, we decide to remove DEFAULT value in terminal since it is confusing as default in TPOT. And user can easily use config_dict parameter to add default value back based on scikit-learn online documentation.

[coveralls]

Coverage decreased (-0.1%) to 88.288% when pulling 7c44a67 on weixuanfu2016:issue499 into c427093 on rhiever:development.