[dev app] add remove tag

* Adding tags with blank value will remove the tag

Author: nan-li

iOS_SDK/OneSignalDevApp/OneSignalDevApp/ViewController.m

@@ -121,10 +121,12 @@ - (IBAction)removeAliasButton:(UIButton *)sender {
 }
 
 - (IBAction)sendTagButton:(id)sender {
-    if (self.tagKey.text && self.tagKey.text.length
-        && self.tagValue.text && self.tagValue.text.length) {
+    if (self.tagValue.text && self.tagValue.text.length) {
         NSLog(@"Sending tag with key: %@ value: %@", self.tagKey.text, self.tagValue.text);
         [OneSignal.User addTagWithKey:self.tagKey.text value:self.tagValue.text];
+    } else {
+        NSLog(@"Removing tag with key: %@", self.tagKey.text);
+        [OneSignal.User removeTag:self.tagKey.text];
     }
 }