Update jsb_basic_conversions.mm

fixed rounding bug in JSB_jsval_from_NSNumber

JSB_jsval_to_NSSet now uses a real NSSet

in JSB_jsval_to_NSDictionary the key can be a number or a string
-> now JSB_jsval_from_NSDictionary also
Update jsb_basic_conversions.mm

if a jsvalue is null, it should generate a [NSNull null]
JSB_jsval_from_unknown works in the same way with "return JSVAL_NULL"
NSJSONSerialization also geneates a NSNull from a "null" js value
Merge pull request #66 from many20/master

update for Update jsb_basic_conversions.mm