Add model for String.valueOf(boolean)

Author: romainbrenguier

src/main/java/java/lang/String.java

@@ -3873,9 +3873,7 @@ public static String copyValueOf(char data[]) {
      * @diffblue.untested
      */
     public static String valueOf(boolean b) {
-        // DIFFBLUE MODEL LIBRARY This is treated internally in CBMC
-        return CProver.nondetWithoutNullForNotModelled();
-        // return b ? "true" : "false";
+        return b ? "true" : "false";
     }
 
     /**