.Net core support and minor fixes

.Net core support and minor fixes:

1. .Net Core support!
2. Mapper.RegisterCustom is not used when source value is default.
3. Exception Message Typo.
4. When the member names are the same configuration is ignored.
5. Guid NULL Check

Here are the details: https://github.com/fluentsprings/ExpressMapper/issues?q=is%3Aissue+milestone%3A1.8.2+is%3Aclosed