add 1 before fitting final week when creating shifted cases

[sbfnk]

Description

This PR closes #611 by keeping values constant instead of setting zeroes to 1 (this approach has proven unstable) by adding 1 to the number of confirmed cases before fitting (and subtracting again when using the fit).

Initial submission checklist

• My PR is based on a package issue and I have explicitly linked it.
• I have tested my changes locally (using devtools::test() and devtools::check()).
• I have added or updated unit tests where necessary.
• I have updated the documentation if required and rebuilt docs if yes (using devtools::document()).
• I have followed the established coding standards (and checked using lintr::lint_package()).
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After the initial Pull Request

• I have reviewed Checks for this PR and addressed any issues as far as I am able.

[jamesmbaazam]

This LGTM. I wonder if this feature should be exposed in the function's documentation?

[sbfnk]

This LGTM. I wonder if this feature should be exposed in the function's documentation?

Thanks, added some detail in 2ab300e and used the opportunity to improve the example and code documentation (with apologies, slightly deviating from the original Issue).