Add checkstyle rule for StringBuilder.append(char)

And replace all instances where String.Builder.append() is called with a
String of length one with append(char).

Also adds StringUtils.toStringBuilder(Collection, String).

smack-extensions/src/main/java/org/jivesoftware/smackx/commands/packet/AdHocCommandData.java

@@ -261,7 +261,7 @@ public class AdHocCommandData extends IQ {
 
         public String toXML() {
             StringBuilder buf = new StringBuilder();
-            buf.append("<").append(getElementName());
+            buf.append('<').append(getElementName());
             buf.append(" xmlns=\"").append(getNamespace()).append("\"/>");
             return buf.toString();
         }