title: How to resolve Rack Lint errors?
body: <p>I have noticed that certain requests, particularly from Uptime Robot to test page uptime have recently started generated the following Rack Lint error on my Rails 3.1.x app:</p>
  
  <pre><code>21:47:05 web.1     | Started HEAD "/" for 74.86.158.106 at 2012-03-22 21:47:05 -0400
  21:47:05 web.1     |   Processing by HomeController#index as */*
  ...
  21:47:05 web.1     | /Users/username/.rvm/gems/ruby-1.9.3-p125@project/gems/activesupport-3.1.4/lib/active_support/core_ext/string/output_safety.rb:23: warning: regexp match /.../n against to UTF-8 string
  ...
  21:47:05 web.1     | Completed 200 OK in 81ms (Views: 60.3ms | ActiveRecord: 0.0ms)
  21:47:05 web.1     | 74.86.158.106 - - [22/Mar/2012 21:47:05] "GET / HTTP/1.1" 200 31842 0.1993
  21:47:05 web.1     | E, [2012-03-22T21:47:05.546848 #18938] ERROR -- : app error: Content-Length header was 31842, but should be 0 (Rack::Lint::LintError)
  21:47:05 web.1     | E, [2012-03-22T21:47:05.546978 #18938] ERROR -- : /Users/username/.rvm/gems/ruby-1.9.3-p125@project/gems/rack-1.3.6/lib/rack/lint.rb:19:in `assert'
  21:47:05 web.1     | E, [2012-03-22T21:47:05.547079 #18938] ERROR -- : /Users/username/.rvm/gems/ruby-1.9.3-p125@project/gems/rack-1.3.6/lib/rack/lint.rb:501:in `verify_content_length'
  21:47:05 web.1     | E, [2012-03-22T21:47:05.547178 #18938] ERROR -- : /Users/username/.rvm/gems/ruby-1.9.3-p125@project/gems/rack-1.3.6/lib/rack/lint.rb:525:in `each'
  21:47:05 web.1     | E, [2012-03-22T21:47:05.547277 #18938] ERROR -- : /Users/username/.rvm/gems/ruby-1.9.3-p125@project/gems/rack-1.3.6/lib/rack/body_proxy.rb:23:in `method_missing'
  21:47:05 web.1     | E, [2012-03-22T21:47:05.547375 #18938] ERROR -- : /Users/username/.rvm/gems/ruby-1.9.3-p125@project/gems/unicorn-4.2.0/lib/unicorn/http_response.rb:41:in `http_response_write'
  21:47:05 web.1     | E, [2012-03-22T21:47:05.547476 #18938] ERROR -- : /Users/username/.rvm/gems/ruby-1.9.3-p125@project/gems/unicorn-4.2.0/lib/unicorn/http_server.rb:538:in `process_client'
  21:47:05 web.1     | E, [2012-03-22T21:47:05.547574 #18938] ERROR -- : /Users/username/.rvm/gems/ruby-1.9.3-p125@project/gems/unicorn-4.2.0/lib/unicorn/http_server.rb:604:in `worker_loop'
  21:47:05 web.1     | E, [2012-03-22T21:47:05.547672 #18938] ERROR -- : /Users/username/.rvm/gems/ruby-1.9.3-p125@project/gems/unicorn-4.2.0/lib/unicorn/http_server.rb:487:in `spawn_missing_workers'
  21:47:05 web.1     | E, [2012-03-22T21:47:05.547770 #18938] ERROR -- : /Users/username/.rvm/gems/ruby-1.9.3-p125@project/gems/unicorn-4.2.0/lib/unicorn/http_server.rb:137:in `start'
  21:47:05 web.1     | E, [2012-03-22T21:47:05.547867 #18938] ERROR -- : /Users/username/.rvm/gems/ruby-1.9.3-p125@project/gems/unicorn-4.2.0/bin/unicorn:121:in `&lt;top (required)&gt;'
  21:47:05 web.1     | E, [2012-03-22T21:47:05.547966 #18938] ERROR -- : /Users/username/.rvm/gems/ruby-1.9.3-p125@project/bin/unicorn:19:in `load'
  21:47:05 web.1     | E, [2012-03-22T21:47:05.548063 #18938] ERROR -- : /Users/username/.rvm/gems/ruby-1.9.3-p125@project/bin/unicorn:19:in `&lt;main&gt;'
  </code></pre>
  
  <p>Any ideas as to what may be going on here?  I'm really not finding a solution right now that specifically addresses what to do in this scenario as far as poking and hunting down what really may be going on.</p>