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.

src/test/java/org/gaul/s3proxy/S3ProxyTest.java

@@ -481,9 +481,6 @@ public final class S3ProxyTest {
 
         for (int i = 0; i < numParts; ++i) {
             ByteSource partByteSource = byteSource.slice(i, 1);
-            // TODO: wrap sliced byte source to work around zero length bug
-            partByteSource = ByteSource.wrap(partByteSource.read());
-            assertThat(partByteSource.size()).isEqualTo(1);  // TODO:
             Payload payload = Payloads.newByteSourcePayload(partByteSource);
             payload.getContentMetadata().setContentLength(
                     partByteSource.size());

src/test/java/org/gaul/s3proxy/TestUtils.java

@@ -75,7 +75,8 @@ final 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