add tests to confirm the behavior when `format` and `extensions` are conflicted

[haoxins]

If we have the file structure below

/hello
/hello.html
/hello/index.html

with options

{
  index: 'index.html',
  extensions: ['.html']
}

[Necromos]

@coderhaoxin
Hmmm after first look I found out that:

1. GET /hello:
   /hello.html is served (which in my opinion is correct but I would like to hear yours)

2. GET /hello.html:
   hello.html is served (expected) even if options is set to:

{ 
  index: 'index.html',
  extensions: ['.txt', '.html'] 
}

3. GET /hello/:
   is serving index.html (expected)

If all is ok then I can push tests for these cases.