Fix nested function (#159)

* switched order in token::kind::id of function and operator behavior

* add tests

* fixed nested function order

* created single include

Co-authored-by: pantor <lars.berscheid@online.de>
diff --git a/include/inja/parser.hpp b/include/inja/parser.hpp
index 348f315..b22e04d 100644
--- a/include/inja/parser.hpp
+++ b/include/inja/parser.hpp
@@ -289,6 +289,9 @@
           }
 
           function_stack.pop();
+          
+          current_expression_list->rpn_output.emplace_back(operator_stack.top());
+          operator_stack.pop();
         }
       }
       default:
diff --git a/single_include/inja/inja.hpp b/single_include/inja/inja.hpp
index ec9937d..45595d0 100644
--- a/single_include/inja/inja.hpp
+++ b/single_include/inja/inja.hpp
@@ -3001,6 +3001,9 @@
           }
 
           function_stack.pop();
+          
+          current_expression_list->rpn_output.emplace_back(operator_stack.top());
+          operator_stack.pop();
         }
       }
       default: