Cocoa: Fix assert on disabling window aspect ratio

When disabling window aspect ratio, a system assert would trigger.  The
correct way to disable window aspect ratio is to set a resize increment.

Closes #852.

src/cocoa_window.m

@@ -1183,7 +1183,7 @@ void _glfwPlatformSetWindowSizeLimits(_GLFWwindow* window,
 void _glfwPlatformSetWindowAspectRatio(_GLFWwindow* window, int numer, int denom)
 {
     if (numer == GLFW_DONT_CARE || denom == GLFW_DONT_CARE)
-        [window->ns.object setContentAspectRatio:NSMakeSize(0, 0)];
+        [window->ns.object setResizeIncrements:NSMakeSize(1.0, 1.0)];
     else
         [window->ns.object setContentAspectRatio:NSMakeSize(numer, denom)];
 }