fix: Fixes asset not defined error when using upload delivery type (#36)

## Description

Fixes a bug that creates an asset not defined error if delivery type is
set to upload.

Tests did not capture this 1) because the upload delivery method tests
were commented out (by me) until we can mock them and 2) because we were
not testing any use cases where we passed in the `assets` data into the
update function.

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## Issue Ticket Number
Fixes #35

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## Type of change
<!-- Please select all options that are applicable. -->
- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [ ] This change requires a documentation update

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# Checklist:
- [x] I have followed the contributing guidelines of this project as
mentioned in [CONTRIBUTING.md](/CONTRIBUTING.md)
- [x] I have created an
[issue](https://github.com/colbyfayock/netlify-plugin-cloudinary/issues)
ticket for this PR
- [x] I have checked to ensure there aren't other open [Pull
Requests](https://github.com/colbyfayock/netlify-plugin-cloudinary/pulls)
for the same update/change?
- [x] I have performed a self-review of my own code
- [x] I have run tests locally to ensure they all pass
- [x] I have commented my code, particularly in hard-to-understand areas
- [x] I have made corresponding changes needed to the documentation

Author: colbyfayock