When creating content containing a text-line (e.g. a folder) an exception is thrown when the content of the text-line is numeric.
Only occurs when creating content using XML as the requests content.

The checkValueStructure-method of the TextLine-Type is throwing an InvalidArgumentException.
https://github.com/ezsystems/ezpublish-kernel/blob/master/eZ/Publish/Core/FieldType/TextLine/Type.php#L227

This error occurs since numeric values will be returned as integer/double when parsed.
The TextLine-Type may also allow numeric values (e.g. integer or double).