entityhub:text value converter should only convert String values

The Entityhub Fieldmappings do support conversion of values (e.g. parsing Numbers, Date/Time values from string values).

The converter for the entityhub:text datatype (that represents natural lanugage texts) currently support any value type by simple calling the #toString() method on the parsed value. This is not the intension of this converter as it does not make sense to convert e.g. URIs to natural language text.

With this issue the converter will be changed so that only string values are converted to natural language text values (without a language).

Users that want to convert any value to its textual representation can still use the xsd:string converter.