Allow 'object' propType in 'to' prop of 'MenuItemLink'

[afilp]

As documented in react-router, the <NavLink> component can have either string or object type for its `to' property:

https://reacttraining.com/react-router/web/api/Link

Some developers use it as such:

              <MenuItemLink
                to={{
                  pathname: '/departments',
                }}
                primaryText="departments"
                title="departments"
                onClick={onMenuClick}
                leftIcon={<BusinessIcon />}
              />

[fzaninotto]

Thanks!