MINOR use proper template classes for internalSelectKey()

[tedyu]

As pointed out in this thread: http://search-hadoop.com/m/Kafka/uyzND1fy2K7I85G1?subj=Kafka+source+code+Build+Error , Eclipse shows syntax error for the following:

                        return new KeyValue<>(mapper.apply(key, value), value);

[guozhangwang]

Hmm... what Java version could cause the issue?

Type mismatch: cannot convert from KeyValue<capture#27-of ? extends K1,V> to KeyValue<K1,V>

[mjsax]

Just wondering if we should put a workaround to fix Eclipse? I tend to day rather no.

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