Improve type checking in sanitize (#189)

Resolves #160

Author: Shreyas Minocha

VerbalExpressions.js

@@ -40,14 +40,18 @@ class VerbalExpression extends RegExp {
      * @memberof VerbalExpression
      */
     static sanitize(value) {
-        if (value.source) {
+        if (value instanceof RegExp) {
             return value.source;
         }
 
         if (typeof value === 'number') {
             return value;
         }
 
+        if (typeof value !== 'string') {
+            return '';
+        }
+
         // Regular expression to match meta characters
         // https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/regexp
         const toEscape = /([\].|*?+(){}^$\\:=[])/g;

dist/verbalexpressions.js

@@ -632,14 +632,18 @@ var VerbalExpression = function (_extendableBuiltin2) {
     }], [{
         key: 'sanitize',
         value: function sanitize(value) {
-            if (value.source) {
+            if (value instanceof RegExp) {
                 return value.source;
             }
 
             if (typeof value === 'number') {
                 return value;
             }
 
+            if (typeof value !== 'string') {
+                return '';
+            }
+
             // Regular expression to match meta characters
             // https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/regexp
             var toEscape = /([\].|*?+(){}^$\\:=[])/g;