add more tests for extensions option

[haoxins]

• should not search if path has extension
• should skip directories

[Necromos]

@coderhaoxin

• should not search if path has extension
  • It will be pushed with add tests to confirm the behavior when format and extensions are conflicted #66 probably
• should skip directories
  • Well I didn't knew what it should do in this case, right now when GET /someDirectory/ is called it will try to deconstruct path and search for /someDirectory{.extension} which may be not really wanted behaviour. Should we omit extension checking if directory is called?