Set zero values to false

Currently setting a configuration parameter to `false` results in an
error when `Rack::Timeout.set_timeout_property` calls `#zero?` on
`false`. Updating `.set_timeout_property` to only call `#zero?` on the
configuration value if the value is not falsy.
Merge pull request #59 from geoffharcourt/fix-docs-wait-timeout

don't bork when setting a timeout property directly to false.