Check for null target in avoidProxy to prevent exceptions; Close #560.

The error is happening pretty deep in the guts of the code at the point where you already have the url and are trying to execute it (see partial stacktrace below).  Ultimately, I'd be fine with dropping this change and test as long as the changes to AsyncHttpProviderUtils make it in, because then callers can check bad urls ahead of time.  Would you honor the pull request if I dropped ProxyUtils*?

java.lang.NullPointerException
        at com.ning.http.util.ProxyUtils.avoidProxy(ProxyUtils.java:101)
        at com.ning.http.util.ProxyUtils.avoidProxy(ProxyUtils.java:86)
        at com.ning.http.util.ProxyUtils.getProxyServer(ProxyUtils.java:72)
        at
com.ning.http.client.providers.netty.NettyAsyncHttpProvider.doConnect(NettyAsyncHttpProvider.java:955)
        at
com.ning.http.client.providers.netty.NettyAsyncHttpProvider.execute(NettyAsyncHttpProvider.java:937)
        at
com.ning.http.client.AsyncHttpClient.executeRequest(AsyncHttpClient.java:520)