Update json.nim

`%` - Conversion of an object or tuple to JsonNode doesn't work correctly for tuples atm, since the generic type is missing. 
Added the tuple generic type to correctly allow tuples as input as well. 
Credits to Elegantbeef from the Nim Discord.

lib/pure/json.nim

@@ -390,7 +390,7 @@ proc `[]=`*(obj: JsonNode, key: string, val: JsonNode) {.inline.} =
   assert(obj.kind == JObject)
   obj.fields[key] = val
 
-proc `%`*[T: object](o: T): JsonNode =
+proc `%`*[T: object | tuple](o: T): JsonNode =
   ## Construct JsonNode from tuples and objects.
   result = newJObject()
   for k, v in o.fieldPairs: result[k] = %v