Don't die if Rack::Lint is loaded

Rack::Lint is loaded by default in development, and changes the type of
`env['rack.input']` to a Rack::Lint::InputWrapper, which does not respond to
`size`.

Fix copied from:
https://github.com/joshfrench/rack-timeout/commit/06af2cb7f767b94c9c9e777707cdcbf760e8811f