Process PDF URLs when using Gravity Wiz Google Sheets

[jakejackson1]

Description

Users could map PDF URLs in the GWiz Google Sheets feed, but these URLs would not be correctly processed and sent.

Fixes #1491

Testing instructions

In a form with a PDF configured:

1. Add a Google Sheets feed and map the PDF URL, signed PDF URL, and Custom Value with PDF merge tag to columns in the spreadsheet
2. Submit the form

The spreadsheet will now contain the correct PDF URLs.

Screenshots

Checklist:

• [x] I've tested the code.
• [x] My code is easy to read, follow, and understand
• [ ] My code follows the accessibility standards.
• [x] My code has proper inline documentation / docblocks.

Additional Comments

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 77.01%. Comparing base (c7b7dcb) to head (3917f08).

Files	Patch %	Lines
src/Model/Model_Mergetags.php	0.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## development #1499 +/- ## =============================================== - Coverage 77.03% 77.01% -0.02% =============================================== Files 243 243 Lines 12509 12513 +4 Branches 370 370 =============================================== + Hits 9636 9637 +1 - Misses 2865 2868 +3 Partials 8 8 ```

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