Fix Array.appendAll() arraycopy type mismatch

[pivovarit]

Internal Array.appendAll optimizations sometimes would result in returning arrays of the wrong type.

private interface SomeInterface {
}

enum OneEnum implements SomeInterface {
    A1, A2, A3;
}

enum SecondEnum implements SomeInterface {
    A1, A2, A3;
}

public static void main(String[] args) {
    Array<SomeInterface> empty = Array.empty();
    Array<SomeInterface> copy1 = empty.appendAll(Array.<SomeInterface>of(OneEnum.values()));
    Array<SomeInterface> copy2 = copy1.appendAll(Array.<SomeInterface>of(SecondEnum.values()));
}

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fixes: #2781