When prompting, use show_default as the default if its a string

[thejcannon]

This PR just makes sure the default handed to .prompt is the show_default if its a string (to match the help).

fixes #2836

• Add tests that demonstrate the correct behavior of the change. Tests
  should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.

[thejcannon]

oh duh, this changes the default 🤦 . Gotta go deeper...