Fix undefined quote in Item:build_return_groups

got following error:
```
/Users/mikz/Developer/3scale/slug/monitor/lua/http_ng/response.lua:48: internal LDoc error
/usr/local/share/lua/5.2/ldoc/doc.lua:931: attempt to call global 'quote' (a nil value)
stack traceback:
	/usr/local/share/lua/5.2/ldoc/doc.lua:931: in function 'build_return_groups'
	/usr/local/share/lua/5.2/ldoc/doc.lua:758: in function 'finish'
	/usr/local/share/lua/5.2/ldoc/doc.lua:236: in function 'finish'
	/usr/local/share/lua/5.2/ldoc/parse.lua:404: in function </usr/local/share/lua/5.2/ldoc/parse.lua:404>
```