Use `headers` instead of `header`, since in `Rack::Response` `headers` is an alias for `header` and `headers` is widely used in Sinatra. It's confusing that in `Sinatra::Response#finish` both `headers` and `header`.
Fix test failures with RDoc 4 broken by commit rdoc/rdoc@7f0033d
Merge pull request #659 from patriciomacadden/header-headers

Use `headers` method instead of `header` method in `Sinatra::Response`
Convert Chinese README to Markdown
Merge pull request #661 from gcapizzi/chinese_readme_md

Convert Chinese README to Markdown
Merge pull request #660 from bkabrda/master

Fix tests with RDoc 4
test pass with rdoc 4 now
Revert "test pass with rdoc 4 now"

This reverts commit 9f1cdabc88885adcc1c3f49710d4eb138958d1a8.
use rdoc 3.x on 1.8