[SPARK-5317]Set BoostingStrategy.defaultParams With Enumeration Algo.Classification or Algo.Regression

[Peishen-Jia]

JIRA Issue: https://issues.apache.org/jira/browse/SPARK-5317
When setting the BoostingStrategy.defaultParams("Classification"), It's more straightforward to set it with the Enumeration Algo.Classification, just like BoostingStragety.defaultParams(Algo.Classification).
I overload the method BoostingStragety.defaultParams().

[AmplabJenkins]

Can one of the admins verify this patch?

[mengxr]

ok to test

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[mengxr]

This doesn't work in Java. You need at least "Algo.Classification()`.

[Peishen-Jia]

Seems that I am a little careless about the docs of Java version. I will update it.

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Test build #25865 has started for PR 4103 at commit 7c1e6ee.

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Test build #25933 has started for PR 4103 at commit 3b72875.

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Test build #25933 has finished for PR 4103 at commit 3b72875.

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[mengxr]

This is my bad. In Scala, the parameter name is part of the API. So let's not rename algo to algoStr.

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Test build #25943 has started for PR 4103 at commit 87bab1c.

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[mengxr]

LGTM pending Jenkins.

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Test build #25943 has finished for PR 4103 at commit 87bab1c.

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