Timestamp deserialization not working for CSV, Properties or XML

[cowtowncoder]

Use of integer timestamps works for dataformats with integer datatype for scalars, including JSON as well as YAML and all binary formats. However, they do not work for textual formats that do not have native numeric types: CSV, Properties or XML.
With 2.12 it is possible to recognize use with such formats so it should be possible to support case of "timestamp number as String" just for these formats without necessarily extending values to JSON (although support for other formats may be added in configurable way, if users want it).