update StringUtils to apache lang3 #314

Author: code4craft

webmagic-core/src/main/java/us/codecraft/webmagic/downloader/HttpClientDownloader.java

@@ -2,7 +2,7 @@
 
 import com.google.common.collect.Sets;
 import org.apache.commons.io.IOUtils;
-import org.apache.commons.lang.StringUtils;
+import org.apache.commons.lang3.StringUtils;
 import org.apache.http.HttpHost;
 import org.apache.http.HttpResponse;
 import org.apache.http.NameValuePair;