request.rb: Compatibility with rackup's WEBRick handler

Author: olleolleolle

lib/slack/events/request.rb

@@ -39,9 +39,9 @@ def version
       def body
         @body ||= begin
           input = http_request.body
-          input.rewind
+          input.rewind if input.respond_to?(:rewind)
           body = input.read
-          input.rewind
+          input.rewind if input.respond_to?(:rewind)
           body
         end
       end