Fix incorrect effect_learner_objective in XGBRRegressor

[jeongyoonlee]

Proposed changes

This PR fixes #501.

Currently the default value of effect_learner_objective in XGBRRegressor is incorrectly set to rank:pairwise, which is for classification.

This PR updates it to reg:squarederror, and also splits the test for XGBRRegressor with sample_weights from one for BaseRClassifier with sample_weights.

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Further comments

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[paullo0106]

LGTM!