[MRG+1] #3364 warm start in random forests

Related to issue #3364.
As for the 40 samples, it was more a way to make sure that there is enough 'randomness' to get different oob scores if the computation or the forest is different.
If you take too few samples it makes the probability that two different forests give the same oob error higher. 
I don't think it is too relevant though and I am changing n_samples to 20, I just explain it to answer your question.

As for the `astype` I used it because it was used in the existing tests. I think I read somewhere in the doc that it makes some computation faster, but I am not sure it makes sense here.
Should I remove it from my tests and the existing ones?