fix: Add Missing Mapping of DateTime in Multipart

[Sadhorsephile]

Problem

If we have a request like this:

  @MultiPart()
  @POST('/api/some/path/')
  Future<SomeResponse> requestName({
    @Part(name: 'full_name') required String fullName,
    @Part(name: 'birth_date') required DateTime birthDate,

we'll get a toJson() method have to add to DateTime Exception.

Solution

If part param doesn't have the toJson() method, we have to check whether this is DateTime. If so, we use atoIso8601String method.