allow additional attributes in <join> element (trac #1982)

[behrisch]

similar to the way it is done with the <split> element, the node attributes type, tl, tlType, shape, radius, keepClear should be supported to affected the resulting node.

Migrated from http://sumo.dlr.de/ticket/1982

{
    "status": "new", 
    "changetime": "2015-11-03T14:27:28Z", 
    "description": "similar to the way it is done with the <split> element, the node attributes type, tl, tlType, shape, radius, keepClear should be supported to affected the resulting node.", 
    "reporter": "namdre", 
    "cc": "", 
    "resolution": "", 
    "_ts": "1446560848765529", 
    "component": "net building (NETGEN, NETCONVERT)", 
    "summary": "allow additional attributes in <join> element", 
    "priority": "minor", 
    "keywords": "", 
    "time": "2015-11-03T14:27:28Z", 
    "milestone": "1.1.0", 
    "owner": "", 
    "type": "enhancement"
}