bugfix: floating point representaion without fractional part

closes https://github.com/json-c/json-c/issues/278

json_object.c

@@ -693,6 +693,7 @@ static int json_object_double_to_json_string_format(struct json_object* jso,
 {
   char buf[128], *p, *q;
   int size;
+  double dummy; /* needed for modf() */
   /* Although JSON RFC does not support
      NaN or Infinity as numeric values
      ECMA 262 section 9.8.1 defines
@@ -706,7 +707,9 @@ static int json_object_double_to_json_string_format(struct json_object* jso,
       size = snprintf(buf, sizeof(buf), "-Infinity");
   else
     size = snprintf(buf, sizeof(buf),
-        format ? format : "%.17g", jso->o.c_double);
+        format ? format : 
+          (modf(jso->o.c_double, &dummy) == 0) ? "%.17g.0" : "%.17g",
+          jso->o.c_double);
 
   p = strchr(buf, ',');
   if (p) {

tests/test_parse.expected

@@ -31,8 +31,8 @@ new_obj.to_string()={ "foo": "bar", "baz": null, "bool0": true }
 new_obj.to_string()={ "foo": [ null, "foo" ] }
 new_obj.to_string()={ "abc": 12, "foo": "bar", "bool0": false, "bool1": true, "arr": [ 1, 2, 3, null, 5 ] }
 new_obj.to_string()={ "abc": "blue\nred\ngreen" }
-new_obj.to_string()=[ 0 ]
-new_obj.to_string()=[ 0 ]
+new_obj.to_string()=[ 0.0 ]
+new_obj.to_string()=[ 0.0 ]
 new_obj.to_string()=null
 new_obj.to_string()=[ 9223372036854775807 ]
 ==================================