Normative: Make JSON.stringify(-0) preserve the sign

Although JSON supports -0 per ECMA-404 and JSON.parse('-0') returns -0, JSON.stringify(-0) currently loses the sign, outputting just '0'.

This patch makes JSON.stringify(-0) return '-0' instead, so that the following holds:

    Object.is(JSON.parse('-0'), -0);
    // → true (this is already the case, even without this patch)
    JSON.stringify(-0);
    // → '-0'
    Object.is(JSON.parse(JSON.stringify(-0)), -0);
    // → true

Results in current JavaScript engines:

    $ eshost -se 'Object.is(JSON.parse("-0"), -0)'
    #### Chakra, JavaScriptCore, SpiderMonkey, V8
    true

    #### XS
    false

    $ eshost -se 'JSON.stringify(-0)'
    #### Chakra, JavaScriptCore, SpiderMonkey, V8, XS
    0

    $ eshost -se 'Object.is(JSON.parse(JSON.stringify(-0)), -0)'
    #### Chakra
    true

    #### JavaScriptCore, SpiderMonkey, V8, XS
    false

Author: mathiasbynens

spec.html

@@ -36323,6 +36323,7 @@ <h1>Runtime Semantics: SerializeJSONProperty ( _key_, _holder_ )</h1>
           1. If _value_ is *false*, return `"false"`.
           1. If Type(_value_) is String, return QuoteJSONString(_value_).
           1. If Type(_value_) is Number, then
+            1. If _value_ is *-0*, return `"-0"`.
             1. If _value_ is finite, return ! ToString(_value_).
             1. Return `"null"`.
           1. If Type(_value_) is Object and IsCallable(_value_) is *false*, then