Parse JSON numbers to Number type instead of Double.

[zenglian]

Currently all numbers are parsed as Double thus integers are parsed as "int.0", which is wrong according to JSON specification (see www.json.org). With the spec, JSON's number is defined as "int or
int.frac or scientific annotation". This fix parses JSON numbers to Java's Number using NumberFormat parser, which correctly parses JSON numbers to Long and Double respectively. This resolves a lot of issues raised in the past years and also correctly parse "9223372036854775807" to Long.MAX_VALUE without precision loss.

[zenglian]

    public void testNumber() {
        Gson gson = new Gson();
        Object[] numbers1 = new Object[]{1, 1.0, 1.23, 9223372036854775807L, new BigInteger("9223372036854775808"),
                1.23E10, 1.23E20};
        String json = gson.toJson(numbers1);
        Object[] numbers2 = gson.fromJson(json, Object[].class);
        for (int i = 0; i < numbers2.length; i++) {
            System.out.println(numbers1[i] + " (" + numbers1[i].getClass().toString().substring(6)
                    + ")\t-->\t" + numbers2[i] + " (" + numbers2[i].getClass().toString().substring(6) + ")");
        }
    }

//result
1 (java.lang.Integer)	-->	1 (java.lang.Long)
1.0 (java.lang.Double)	-->	1 (java.lang.Long)
1.23 (java.lang.Double)	-->	1.23 (java.lang.Double)
9223372036854775807 (java.lang.Long)	-->	9223372036854775807 (java.lang.Long)
9223372036854775808 (java.math.BigInteger)	-->	9.223372036854776E18 (java.lang.Double)
1.23E10 (java.lang.Double)	-->	12300000000 (java.lang.Long)
1.23E20 (java.lang.Double)	-->	1.23E20 (java.lang.Double)

[zenglian]

JSON does have "int" (the same as Java though there is no restriction on its size/no base8/16), see www.json.org.

...
number -> int | int frac | int exp | int frac exp
...

[astux7]

Can this PR be merge asap - it is an issue when i am trying to use json for long?