Elements data empty when form_key contains uppercase characters in D7 webform submissions

This patch makes the element's form_key lowercase in the `d7_webform_submission` source plugin. Please review.

I was going to provide a patch myself for this bug, but I came across this issue where #2 patch resolves it.

I confirm the bug is still present in the latest version of webform submission migration from D7, and it is caused because of applying the strtolower() function to the 'form_key' in the webform migration source plugin, but not in the webform submission source plugin (this difference makes the 'form_key' inconsistent).

When executed, the migration doesn't fail per-se (you can actually check that the submission data is present in the database), however the UI is not able to properly display the submissions for those items where the 'form_key' originally contained uppercase characters. The provided patch also applies the strtolower() to the 'form_key' in the webform submission source plugin and fixes the issue. Thank you.