Fixed issue #417

[zsxwing]

public static void main(String[] args) {
    System.out.println("started");
    System.out.println(Observable.from(1).observeOn(Schedulers.threadPoolForComputation())
        .map(f).toBlockingObservable().single());
    System.out.println("done");
}

This issue in #417 is because current map implementation violates the design guild:

6.4. Protect calls to user code from within an operator

Now current map does not handle the error from the func param which may be user codes. When the thread pool scheduler is involved, the error from func will throw to the outside and be handled by java.util.concurrent.FutureTask. The error will be dropped so the observer will not see it.

This PR used SafeObserver to protect map from user codes.

[cloudbees-pull-request-builder]

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[benjchristensen]

Thank you @zsxwing