Add default user agent (#510)

Author: dr0i

metafacture-io/src/main/java/org/metafacture/io/SruOpener.java

@@ -49,7 +49,7 @@ public final class SruOpener extends DefaultObjectPipe<String, ObjectReceiver<Re
 
     private static final String OPERATION = "searchRetrieve";
     private static final String RECORD_SCHEMA = "MARC21-xml";
-    private static final String USER_AGENT = "";
+    private static final String USER_AGENT = "metafacture-core";
     private static final String VERSION = "2.0";
 
     private static final int CONNECTION_TIMEOUT = 11000;