Correct RandomByteSource.read return value

Previously read returned a value between -128 and 127.  -1 indicates
end of stream, causing issues for callers.  Instead return values
between 0 and 255 as intended.

core/src/test/java/org/jclouds/utils/TestUtils.java

@@ -70,7 +70,8 @@ public class TestUtils {
          if (closed) {
             throw new IOException("Stream already closed");
          }
-         return (byte) random.nextInt();
+         // return value between 0 and 255
+         return random.nextInt() & 0xff;
       }
 
       @Override