How to mock a private method using powermock with one of the parameter as byte array

Question

I am trying to mock one of my private method , it is a must for me to mock it . Can any one help me on this . Because my code tries to traverses through the code .I dont need to traverse through the method . I just need to mock my private method and returns a String . Below is my method

Please note : I have referred all the duplicate marked questions, I tried all those and I am unable to find a solution. Please help. Also one of my parameter is a byte array .

private String decodeResponse(byte bresp[])
    {

        String spresp = null;

        byte bcontent[] = new byte[bresp.length - 2];

        for(int i = 0; i < bcontent.length; i++)
            bcontent[i] = bresp[i + 1];

        try
        {
            spresp = new String(bcontent, "US-ASCII");
        }
        catch(UnsupportedEncodingException ex)
        {
            return null;
        }

        return spresp;
    }

Below is my test class

@PrepareForTest(SPSocketCommunicator.class)
@RunWith(PowerMockRunner.class)
public class SPSocketCommunicatorTest{

      @test
        public void test() throws Exception {
        byte[] temp = null;
        SPSocketCommunicator socketCommunicator=new SPSocketCommunicator(request, Map);
                SPSocketCommunicator spy = PowerMockito.spy(socketCommunicator);
        PowerMockito.spy(SPSocketCommunicator.class);
                PowerMockito.doReturn("a string").when(spy,"decodeResponse",Matchers.<byte[]>any());
        socketCommunicator.decodeResponse(temp);
        }
}