completely rewritten pingGoogle / pingBing methods ... tweaks

Author: jirka.pinkas@gmail.com

README.md

@@ -101,7 +101,7 @@ SitemapGenerator.of("https://example.com")
     // generate sitemap and save it to file ./sitemap.xml
     .toFile(Paths.get("sitemap.xml"))
     // inform Google that this sitemap has changed
-    .pingGoogle(); // this requires okhttp in classpath!!!
+    .ping(SearchEngine.GOOGLE); // this requires okhttp in classpath!!!
 ```
 
 ### How to create sitemap index:

src/main/java/cz/jiripinkas/jsitemapgenerator/AbstractSitemapGenerator.java

@@ -210,6 +210,9 @@ public T toFile(File parent, String child) throws IOException {
      * @param sitemapUrl   sitemap url
      */
     public void ping(String sitemapUrl, SearchEngine ... searchEngines) {
+        if(searchEngines.length == 0) {
+            throw new UnsupportedOperationException("Must provide at least one search engine!");
+        }
         for (SearchEngine searchEngine : searchEngines) {
             ping(searchEngine.getPingUrl(), sitemapUrl, searchEngine.getPrettyName());
         }
@@ -225,6 +228,9 @@ public void ping(String sitemapUrl, SearchEngine ... searchEngines) {
      * @return If operation succeeded
      */
     public boolean ping(boolean doNotThrowExceptionOnFailure, SearchEngine ... searchEngines) {
+        if(searchEngines.length == 0) {
+            throw new UnsupportedOperationException("Must provide at least one search engine!");
+        }
         try {
             ping(searchEngines);
             return true;
@@ -247,6 +253,9 @@ public boolean ping(boolean doNotThrowExceptionOnFailure, SearchEngine ... searc
      * @return If operation succeeded
      */
     public boolean ping(String sitemapUrl, boolean doNotThrowExceptionOnFailure, SearchEngine... searchEngines) {
+        if(searchEngines.length == 0) {
+            throw new UnsupportedOperationException("Must provide at least one search engine!");
+        }
         try {
             ping(sitemapUrl, searchEngines);
             return true;
@@ -264,6 +273,9 @@ public boolean ping(String sitemapUrl, boolean doNotThrowExceptionOnFailure, Sea
      * @param searchEngines Search engines to ping
      */
     public void ping(SearchEngine ... searchEngines) {
+        if(searchEngines.length == 0) {
+            throw new UnsupportedOperationException("Must provide at least one search engine!");
+        }
         ping(baseUrl + "sitemap.xml", searchEngines);
     }