Fix issue where changing the customer email results in an error

[kraken-chris]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message is descriptive of the changes within (see https://keepachangelog.com/en/1.1.0/)
• Code conforms to Magento coding standards
• Code is compatible with all currently supported versions of Magento and PHP

PR Type

What kind of change does this PR introduce?

[X] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Fixes: Issue 20

What is the new behavior?

Email is fetch from request first (guest), then from quote (customer).

Does this PR introduce a breaking change?

[ ] Yes
[X] No

Other information