Ctrl+Shift+Equals zooms in but fails to suppress the `+` keystroke

Ctrl+Shift+= on an en_US keyboard (recall that Shift+= generates a +) increases the zoom level as expected but now fails to suppress the + character.

Steps to reproduce:

1. Use a standard en_US qwerty keyboard
2. Open xfce4-terminal version 0.9.1
3. Press Ctrl+Shift+=
4. Repeat step 3 until you get to the maximum zoom size
5. Repeat step 3 one more time

Expected results:

• The terminal zooms incrementally until hitting the max size
• No keystrokes are sent to the shell or other application running in the terminal
• Nothing happens on step 5

Actual results:

• The terminal zooms incrementally until hitting the max size
• One + is added for each zoom-in action triggered by this keyboard shortcut
• Nothing happens on step 5 (not even errant + keystrokes)

This is not an issue with the newly-introduced keypad shortcut (Ctrl+KP_Add) from !26 (merged), and Ctrl+Shift+KP_Add does nothing (as expected).

I'm using Debian Testing but I've pulled xfce4-terminal 0.9.1-1 from Experimental and launched it with xfce4-terminal --disable-server. Most of my system is still using older packages (the latest in Debian Testing), so this might be an artifact related to that.

See also #55 (closed), #126 (closed), and !26 (merged).