Fix double wrapping of algebraic model answers

[PhilippImhof]

Until #257, we simply ignored if an algebraic formula was to be wrapped in quotes a second time. With that PR, we started to throw an error, in order to disallow usage of strings by students.

When creating or editing a question, checking the model answers could lead to the formula being wrapped more than once. This PR fixes the problem.

[coveralls]

Pull Request Test Coverage Report for Build 23984182459

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.001%) to 97.498%

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Totals
Change from base Build 23112103126:	-0.001%
Covered Lines:	4598
Relevant Lines:	4716

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💛 - Coveralls