[5.2] Fix an issue with data_set affecting the Validator

[themsaid]

$data = ['companies' => 'acme'];

data_set($data, 'companies.name', 'acme');

The result is:

['companies' => 'acme']

While it should be:

['companies' => ['name' => 'acme']]

This was affecting a case with the Validator as described here #13212

For the validator's initializeAttributeOnData() to work properly it needs to overwrite the data values, so we need to use data_set() instead of data_fill().

[taylorotwell]

@JosephSilber you have any thoughts on this?