Use IsDefined to check for IgnoreAttribute

[leonluc-dev]

See #558

[praeclarum]

Ah, interesting! I never would have guessed people were using inherited attributes. Thanks for the fix!

[leonluc-dev]

It is not a very common use case for normal use, indeed.
It is mostly commonly used with custom property behaviour and extension frameworks. (Ex: https://bitbucket.org/twincoders/sqlite-net-extensions )
To implement their extension code for only certain attributes, the libraries inherit from the ignore attribute to make sure the default sqlite-net code isnt executed for those properties.