Modified geometry() as discussed on #332.

Made it only accept a custom geometry command if the number of arguments is strictly one.

lib/args.js

@@ -255,9 +255,9 @@ module.exports = function (proto) {
 
   // http://www.graphicsmagick.org/GraphicsMagick.html#details-geometry
   proto.geometry = function geometry (width, height, arg) {
-    // If the first argument is a string, this is a custom geometry command.
-    if(typeof width === "string")
-      return this.out("-geometry", width);
+    // If the first argument is a string, and there is only one argument, this is a custom geometry command.
+    if(arguments.length == 1 && typeof arguments[0] === "string")
+      return this.out("-geometry", arguments[0]);
 
     // Otherwise, return a resizing geometry command with an option alrgument.
     return this.out("-geometry", width+'x'+height+(arg||''));