createKey type error when key consist of only digits

[judip]

Q	A
Bug report?	yes
Feature request?	no
BC Break report?	no
RFC?	no
Branch?	master

In function createKey() in file Importer.php (line 582) should be convert to string:
return (string) $this->expressionLanguage->evaluate(substr($definition->getKey(), 1), $data);

When key consist of only digits is problem because key is not string, but function return type string.

[dpfaffenbauer]

please create a pr