fix: ensure http calls to Wikimedia has an User-agent

src/main/java/org/openrefine/extensions/commons/importer/FileFetcher.java

@@ -43,7 +43,7 @@ public FileFetcher(String apiUrl, String categoryName, boolean subcategories) {
      */
     public void getCallResults(String category) throws IOException {
 
-        OkHttpClient client = new OkHttpClient.Builder().build();
+        OkHttpClient client = HttpClient.getClient();
         urlBase = HttpUrl.parse(apiUrl).newBuilder()
                 .addQueryParameter("action", "query")
                 .addQueryParameter("list", "categorymembers")

src/main/java/org/openrefine/extensions/commons/importer/HttpClient.java

@@ -0,0 +1,35 @@
+package org.openrefine.extensions.commons.importer;
+
+import java.io.IOException;
+
+import okhttp3.Interceptor;
+import okhttp3.OkHttpClient;
+import okhttp3.Request;
+import okhttp3.Response;
+
+public class HttpClient {
+
+    private static final String USER_AGENT = "OpenRefine-Commons-Extension/0.1.* (https://github.com/OpenRefine/CommonsExtension)";
+
+    public static OkHttpClient getClient() {
+        OkHttpClient client = new OkHttpClient.Builder().addInterceptor(new UserAgentInterceptor(USER_AGENT)).build();
+        return client;
+    }
+
+    static class UserAgentInterceptor implements Interceptor {
+
+        private final String userAgent;
+
+        public UserAgentInterceptor(String userAgent) {
+            this.userAgent = userAgent;
+        }
+
+        @Override
+        public Response intercept(Chain chain) throws IOException {
+            return chain.proceed(chain.request().newBuilder()
+                    .header("User-Agent", userAgent)
+                    .build());
+        }
+    }
+    private static OkHttpClient client;
+}

src/main/java/org/openrefine/extensions/commons/importer/RelatedCategoryFetcher.java

@@ -60,7 +60,7 @@ public List<List<String>> getRelatedCategories(List <FileRecord> fileRecordOrigi
         while (titlesIndex < fileRecordOriginal.size()) {
             titles += "|" + fileRecordOriginal.get(titlesIndex++).fileName;
         }
-        OkHttpClient client = new OkHttpClient.Builder().build();
+        OkHttpClient client = HttpClient.getClient();
         HttpUrl urlRelatedCategoriesBase = HttpUrl.parse(apiUrl).newBuilder()
                 .addQueryParameter("action", "query")
                 .addQueryParameter("prop", "categories")