Fix regex escaping bug
Created originally on Bitbucket by evansd (Dave Evans)
Changesets for this Pull Request have not been imported, because it had been already declined on Bitbucket. Marked as closed by the import user.
Sheet names are not escaped before being used to construct a regex in Worksheet._unique_sheet_name
. This means that sheet names such as "Test (" will cause it to break with an unbalanced parentheses error.