Keywords should be OK as object keys (fix #794)

With this change it's now OK to use keywords as object keys like so:

    {if:0}

tests/all.test

@@ -1185,3 +1185,7 @@ try join(",") catch .
 ["1","2",{"a":{"b":{"c":33}}}]
 "string (\",\") and object ({\"a\":{\"b\":{...) cannot be added"
 
+{if:0,and:1,or:2,then:3,else:4,elif:5,end:6,as:7,def:8,reduce:9,foreach:10,try:11,catch:12,label:13,import:14,module:15}
+null
+{"if":0,"and":1,"or":2,"then":3,"else":4,"elif":5,"end":6,"as":7,"def":8,"reduce":9,"foreach":10,"try":11,"catch":12,"label":13,"import":14,"module":15}
+