Incorrectly defaults branch to 'master' when the default branch is set to another branch

[bobalicious]

• Source repo has a default branch that is not master (e.g. main)
• Button code is generated without a branch defined:

E.g.

<a href="https://githubsfdeploy.herokuapp.com">
  <img alt="Deploy to Salesforce"
       src="https://raw.githubusercontent.com/afawcett/githubsfdeploy/master/deploy.png">
</a>

• On launching from the source repo, on the default branch, the 'Branch/Tag/Commit' field is empty (expected)
• On attempting to deploy, the deployment fails with:
  • Could not find the repository 'repo-name'. Ensure it is spelt correctly and that it is owned by 'owner-name'

In the 'From GitHub Repository' it can be seen that the URL is set to include the branch name 'master'