issue-316: Fixed issue where TextField placeholder could be updated while a text value exists.

Sources/iOS/TextField.swift

@@ -379,6 +379,7 @@ public class TextField : UITextField {
 			if let v: String = placeholder {
 				attributedPlaceholder = NSAttributedString(string: v, attributes: [NSForegroundColorAttributeName: placeholderTextColor])
 			}
+			placeholderText = placeholder
 		}
 	}