Redirect subdir with trailing slash.

[phatpaul]

httpd-espfs.c :: tryOpenIndex() already tries to find and serve the content of index.html if the client requests a sub-directory.
This can result in the browser showing the content of /hello/index.html, but the browser's URL is http://blahblah/hello

If /hello/ is a directory, GET /hello should redirect to GET /hello/ to maintain relative paths for the browser.

We should automatically add a trailing slash so clients can properly resolve relative paths.
I.e. GET /hello should redirect to /hello/ because /hello/index.html might require ./app.js, which should resolve to /hello/app.js, NOT /app.js

The exception to this is the root directory does not require a trailing slash.
I.e. GET myapp.com can serve /index.html