update for Update jsb_basic_conversions.mm

fixed rounding bug in JSB_jsval_from_NSNumber
this bug results in a wrong number in js
1.4 (native) -> 1.400000024556 (js)

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

JSB_jsval_to_unknown:
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
If a js value in a dictionary is null it should not be skipped I think.
Null is also a information that can be handled and make sense same times.