Java: Move method StringUtil.toIndentedString to model

Closes #1701

modules/swagger-codegen/src/main/resources/Java/StringUtil.mustache

@@ -39,13 +39,4 @@ public class StringUtil {
     }
     return out.toString();
   }
-
-  /**
-   * Convert the given object to string with each line indented by 4 spaces
-   * (except the first line).
-   */
-  public static String toIndentedString(Object o) {
-    if (o == null) return "null";
-    return o.toString().replace("\n", "\n    ");
-  }
 }