fix(bug): by.buttonText() should not be effected by CSS style

Closes issue #1904

Author: sjelin

lib/clientsidescripts.js

@@ -482,7 +482,7 @@ functions.findByButtonText = function(searchText, using) {
     var element = elements[i];
     var elementText;
     if (element.tagName.toLowerCase() == 'button') {
-      elementText = element.innerText || element.textContent;
+      elementText = element.textContent || element.innerText || '';
     } else {
       elementText = element.value;
     }
@@ -511,7 +511,7 @@ functions.findByPartialButtonText = function(searchText, using) {
     var element = elements[i];
     var elementText;
     if (element.tagName.toLowerCase() == 'button') {
-      elementText = element.innerText || element.textContent;
+      elementText = element.textContent || element.innerText || '';
     } else {
       elementText = element.value;
     }