Speedup PKCS signing using nil as rand io.Reader

With rand.Reader

    PASS
    BenchmarkSignCustomPolicy            100          10536927 ns/op
    ok      /sc-gaws/aws/cloudfront    1.930s

With nil instead of rand.Reader

    PASS
    BenchmarkSignCustomPolicy            200           7775237 ns/op
    ok     /sc-gaws/aws/cloudfront    3.284s