Mouse.isPressed() now checks only for left button by default

if no argument is given now checks left button by default to be consistent with press() and release() (thanks, David Mellis)

cores/arduino/USBAPI.h

@@ -65,7 +65,7 @@ public:
 	void move(signed char x, signed char y, signed char wheel = 0);	
 	void press(uint8_t b = MOUSE_LEFT);		// press LEFT by default
 	void release(uint8_t b = MOUSE_LEFT);	// release LEFT by default
-	bool isPressed(uint8_t b = MOUSE_ALL);	// check all buttons by default
+	bool isPressed(uint8_t b = MOUSE_LEFT);	// check LEFT by default
 };
 extern Mouse_ Mouse;