Makes Grack::Server thread-safe

Author: zimbatm

Diff for: lib/grack/server.rb

@@ -35,6 +35,10 @@ def set_config_setting(key, value)
     end
 
     def call(env)
+      dup._call(env)
+    end
+
+    def _call(env)
       @env = env
       @req = Rack::Request.new(env)