Substitution of formulas

String, started with '=' is considered as formula.
Formulas must be presented in EN(US) equivalent, as it always stored in OOXML data format.

lib/index.js

@@ -736,7 +736,7 @@ module.exports = (function() {
           var formula = new etree.Element("f");
           formula.text = substitution.substr(1);
           cell.insert(1, formula);
-          delete cell.attrib.t;
+          delete cell.attrib.t;  //cellValue will be deleted later
           return formula.text
         }