Fix assertCompilationDiagnostingOn so that it doesn't improperly fail

This should fix an issue where org.androidannotations.rest.RestConverterTest would fail improperly.

When calling assertCompilationDiagnostingOn with @Rest the diagnostics class was returning contentInError like so:
@Rest(converters = { AbstractConverter.class })
@Rest(converters = { Object.class })
@Rest(converters = { WrongConstructorConverter.class })
But the test expected @Rest

There may be a better way to fix this but this got the job done.