make min_density parameter for gradient boosting

I'm not sure why min_density is handled differently for gradient boosting compared to the random forest estimators. This commit simply makes gradient boosting treat min_density the same as random forests. I ran into this in a recent Kaggle competition, I set max_depth < 6 and training slowed to a crawl. Setting min_density to 0.1 fixed that. I'm not sure if the tests are still necessary but I updated them regardless.

I don't know the history of this, but perhaps there was a good reason, @pprett?
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