Update .yardopts to markdown

Update .yardopts file to reflect new use of markdown in docs.
Make the request accept */* when HTTP_ACCEPT is not present or blank
According to https://github.com/slim-template/slim/blob/master/CHANGES
`attr_wrapper` has been renamed to `attr_quote`

Fix slim tests to handle this change
Don't delete Content-Length header if it's a head request or if the body is a Rack::File
Remove trailing whitespaces :cop: [ci skip]
Add integration tests for sending files (Content-Length response header related tests)
Added Reel (https://github.com/celluloid/reel-rack/) as possible Rack Handler.
Renamed URI constant to URI_INSTANCE
update item of route
Cleanup `public_dir` handling
Translate for addition
delete README.jp.md.bak
Changed `map` to `map!` at various places to avoid extra array creation.
Remove explicit block passing
@mongrelion’s spanish README improvements

Instalá el gem y corré la aplicación con → Instalar la gema y correr la aplicación con
Podés verla en http://localhost:4567 → Ver en http://localhost:4567
:traps setting (on by default)
Don’t delete Content-Length if streaming
Test added
#send_file indifferent :status option
Use #Integer (thank you, @rkh)