Xls Writer Treat Hyperlink Starting with # as Internal

[oleibman]

Fix #56, which had gone stale but is now reopened. The problem which it reported with Xlsx Writer was fixed long ago. However, Xls Writer has 2 problems regarding hyperlinks. First, some logic which should have been if ... elseif ... else ... was coded as if ... if ... unconditional ..., resulting in multiple writes and a corrupt worksheet (which Excel does fix correctly). Second, it treated a hyperlink url starting with # (pointer to a cell in the same spreadsheet) as external, when it should be treated as internal.

Also changed Hyperlink::isInternal to recognize a starting #, and to use str_starts_with('sheet://') rather than str_contains.

This is:

• a bugfix
• a new feature
• refactoring
• additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• Documentation is updated as necessary