Handle no-id scene creation properly

[opticron]

This fixes issues to POST /api//scenes that create a new scene without specifying an ID.

[probonopd]

The iOS app with the colorful icon still gets stalled at

=== 166339
Method: GET
requestedUri: api/api/scenes/0
URI: scenes/0
166340
{"name":"Entspannen","owner":"api","picture":"","lastupdated":"","recycle":false,"locked":false,"version":2,"lights":["1","2","3","4","5","6"]}
166349
=== 166384
Method: DELETE
requestedUri: api/api/groups/1
URI: groups/1
166385
=== 168923
Method: GET
requestedUri: api/api/config
URI: config
168925
{"name":"hue emulator","swversion":"81012917","bridgeid":"xxxxxxxxxxxxxxxx","portalservices":false,"linkbutton":true,"mac":"xx:xx:xx:xx:xx:xx","dhcp":true,"ipaddress":"192.168.0.16","netmask":"255.255.255.0","gateway":"192.168.0.1","apiversion":"1.3.0","whitelist":{"":{"name":"clientname#devicename"}},"swupdate":{"text":"","notify":false,"updatestate":0,"url":""}}
168952

[opticron]

It looks like groups delete never responded properly. Another patch incoming.