only reload page when html file changed

Before that if I had `/` opened in browser, it just checked for `/`
being in any path changed on any position except for beginning (0-based
indexing), which means it reloaded a page almost always. Now it appends
`index.html` if a page URI ends with a `/`, which should be pretty
standard.

I don't think it's very important to support cases when people
reconfigure their server to serve some other file name as `index.html`,
but I guess it can be done with an option if wanted.
As an alternative to specifying `index.html` it might also be done more generally for html files:
```
(changed-href? ".html" changed)
```
The reloading will happen whenever an HTML file changed.)