Allow mapping to a dictionary with a key containing periods

[JMolenkamp]

Very basic change to allow for mapping to a dictionary, where the key contains a period. Without this change, config.NewConfig<SimplePoco, Dictionary<string, object>>().Map("Key.With.Periods", c => c.Id); would map to key "Key" instead of "Key.With.Periods".

Simply never splitting in the method CompileArgument.GetDestinationNames does not make any test fail. However, the method is also referenced from ReflectionUtils.ShouldMapMember. It might be that a failing scenario is simply not covered in tests, which is why the option is provided.