Do not require date header w/ none authentication

AWS S3 uses a date header to prevent replay attacks on signed
requests.  None authentication should not require this header.
Regression from 8c623c179c.  Fixes #17.

src/main/java/org/gaul/s3proxy/S3ProxyHandler.java

@@ -144,7 +144,7 @@ final class S3ProxyHandler extends AbstractHandler {
             }
         }
 
-        if (!hasDateHeader && !hasXAmzDateHeader &&
+        if (identity != null && !hasDateHeader && !hasXAmzDateHeader &&
                 request.getParameter("Expires") == null) {
             sendSimpleErrorResponse(response, S3ErrorCode.ACCESS_DENIED);
             baseRequest.setHandled(true);