Fix rack.errors management

[jodosha]

Only dump exceptions in rack.errors if handling is turned off, or the raised exception isn't managed

Closes #74

/cc @joneslee85 and @weppos

[coveralls]

Coverage increased (+0.01%) when pulling fa041b2 on do-not-dump-exception-in-rack-errors-if-it-is-handled into 6c6bbe0 on master.

[weppos]

👍

PS: you could have simply changed the order of the methods from

rescue => exception
  _reference_in_rack_errors(exception)
  _handle_exception(exception)
end

to

rescue => exception
  _handle_exception(exception) || _reference_in_rack_errors(exception)
end

Too magic? 😉

[runlevel5]

👍

[jodosha]

@weppos Three specs fail and yes, it's too magic ;)