Default host to localhost when in development mode.

* Running Rack apps on 0.0.0.0 in development mode will allow malicious
  users on the local network (ex: a Coffee Shop or a Conference) to abuse
  or potentially exploit the app. Safer to default host to localhost when in
  development mode.
Rack::Handler::WEBrick: default the host to localhost in development mode.
Rack::Handler::Mongrel: default the host to localhost in development mode.
Rack::Handler::Thin: default the host to localhost in development mode.
Determine the default value for Host, based on the environment.
add partial rack hijack support for WEBrick
add test for WEBrick partial hijack
Merge pull request #533 from dahakawang/master

add partial rack hijack for WEBrick
Don't rely on ENV. Fixes #532
Removed the comment that rack.pid is the default as it doesn't seem to be the default at all
Merge pull request #535 from oscardelben/fix_env_typo

Don't rely on ENV. Fixes #532
Merge pull request #538 from basvodde/master

Removed default pid file from help text
remove extra loop, variable initialization
Merge pull request #537 from vipulnsward/remove_loop

remove extra loop, variable initialization
fix typos
cleanup tests for unused variables
Merge pull request #539 from vipulnsward/cleanup_tests

cleanup tests for unused variables
Merge pull request #540 from vipulnsward/fix_some_typos

fix typos
Remove unassigned HTTP status codes
Update md5.rb

Separated patch to fix timing attack