Fix none resolution

[medikoo]

I have configuration where I call st multiple times, and following setup lead to errors:

st({ path: '/some/public/path', cache: { fd: false } });
st({ path: '/some/public/path2', cache: false });

Problem is that with first call, internally reusable none object is modified:
https://github.com/isaacs/st/blob/master/st.js#L136
https://github.com/isaacs/st/blob/master/st.js#L157

With second st intialization, dispose goes to all other caches, and that at some point crashes sever:

fs.js:465
  binding.close(fd, req);
          ^
TypeError: Bad argument
    at TypeError (native)
    at Object.fs.close (fs.js:465:11)
    at Object.close (/Users/medikoo/Projects/eregistrations/lomas/node_modules/graceful-fs/graceful-fs.js:37:19)
    at Object.cleanupFd (/Users/medikoo/Projects/eregistrations/lomas/node_modules/st/node_modules/fd/index.js:21:12)
    at Object.<anonymous> (/Users/medikoo/Projects/eregistrations/lomas/node_modules/st/node_modules/fd/index.js:35:21)
    at process._tickCallback (node.js:355:11)

It's caused by stats objects from stat cache being provided to dispose of previous setup fd cache.

I didn't provide test for it, as I'm not that familiar with current test setup, if it's something straightforward please guide and I'll add them.

[rvagg]

oh, good catch, it took me a bit to follow your logic of it being modified but I see the attached dispose and load functions now

[rvagg]

landed as 16c5fe2, published as 0.5.5, cheers

[medikoo]

Thank you!