Fix typo in default value for "webform_datatype" element in webform_handler_field_submission_data class

Problem/Motivation

In the options form of webform_handler_field_submission_data class, the webform_datatype element has its default value set based on the format option.

Proposed resolution

Set the default value based on the webform_datatype option.

Remaining tasks

Review; add test (assume this was not caught due to absence of test)?

User interface changes

None.

API changes

None.