Update 'contains' signature to 'contains(Object)'

[zsxwing]

The original contains signature can not support the following codes:

    public void test() {
        ArrayList<String> l = new ArrayList<String>();
        l.add("test");
        Observable<ArrayList<String>> o = Observable.<ArrayList<String>>from(l);
        o.contains(Arrays.asList("test"));
    }

This PR changes the signature from contains(T) to contains(Object). It's also consistent with equals(Object).

This change should not break the old codes.

[cloudbees-pull-request-builder]

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