ReverseMap creates invalid configuration when Source and Destination support different int values

[sjbuck]

Source/destination types

public enum Source { Default, OnlyInSource = 3 }
public enum Destination { Default, OnlyInDestination = 4}

Mapping configuration

cfg.CreateMap<Source, Destination>()
    .ConvertUsingEnumMapping(opt => opt.MapByValue()
        .MapValue(Source.OnlyInSource, Destination.OnlyInDestination))
    .ReverseMap();

Version: 3.2.0

Expected behavior

• AssertConfigurationIsValid() doesn't throw
• Destination.OnlyInDestination will map to Source.OnlyInSource because of the ReverseMap()

Actual behavior

• AssertConfigurationIsValid() throws
• Trying to map Destination to Source throws

In both cases the error is:

AutoMapper.AutoMapperMappingException : Value OnlyInDestination of type AutoMapper.Extensions.EnumMapping.Tests.ReverseCustomEnumValueMappingByValue+OnlyInSourceValueIsMappedToOnlyInDestiationValue+Destination not supported

Steps to reproduce

// This slightly altered test from the project's test suite will fail
public class OnlyInSourceValueIsMappedToOnlyInDestinationValue : AutoMapperSpecBase
{
    readonly List<Source> _results = new List<Source>();

    public enum Source { Default, OnlyInSource = 3 }
    public enum Destination { Default, OnlyInDestination = 4}

    protected override MapperConfiguration Configuration { get; } = new MapperConfiguration(cfg =>
    {
        cfg.EnableEnumMappingValidation();
        cfg.CreateMap<Source, Destination>()
            .ConvertUsingEnumMapping(opt => opt.MapByValue()
                .MapValue(Source.OnlyInSource, Destination.OnlyInDestination))
            .ReverseMap();
    });
    
    protected override void Because_of()
    {
        _results.Add(Mapper.Map<Destination, Source>(Destination.Default));
        _results.Add(Mapper.Map<Destination, Source>(Destination.OnlyInDestination));
    }

    [Fact]
    public void Should_map_using_custom_map()
    {
        _results[0].ShouldBe(Source.Default);
        _results[1].ShouldBe(Source.OnlyInSource);
    }
}

[lbargaoanu]

A PR is welcome.