As there is no way to copy all the converters of a `ConversionService` to
another, `RelaxedConversionService` uses a fallback `ConversionService`
when the user-provided one failed.
That fallback is taking care of converting `String` to `Enum` in a case
insensitive way but it has no registered converter to convert a comma
separated String to a collection of something.
Ironically, our current test suite has plenty of cases where we map a
`String` to a collection of enums and they all pass. This is because
the tests do not provide a custom `ConverterService` so we end up
immediately in the fallback scenario. Since no converter is able to
convert the String to a collection, the property editor support of the
binder takes care of that for us and try to convert each individual
value.
In a regular use case however, a `ConversionService` is provided and
fails to map the collection if the String value(s) don't have the exact
same case as the annotations they represent. Since the original
`ConversionService` has claimed it was able to convert a collection,
the raw `String` value is passed to the fallback converter and that one
fails to convert the raw String.
The fallback converter now registers the necessary converters to
convert collections. Additional tests have been added to test that in
a more explicit way.
Closes gh-4322
6578239f67