Mpdf Styling of Multi-Line Strings

[oleibman]

Fix #4773. Mpdf is applying cell style only to the first line of a multi-line string. Explanation - Html Writer replaces newlines with <br /> by calling nl2br, at least that was the intention. However, nl2br doesn't replace - it prepends. This doesn't do any harm in Html, Dompdf, or Tcpdf. However, Mpdf is known to be subject to occasional regexp backtracking errors when parsing large blocks of html. To avoid this, we chunk the html by splitting it at newlines, but this causes the styling to be lost for rich text elements following a newline.

The solution to this problem seems easy enough - replace nl2br with a routine that replaces rather than prepends. Unfortunately, Html Reader needs the newlines intact in order to parse things correctly. So a solution where the nl2br calls are used for non-Mpdf and using a substitute routine for Mpdf ought to work. However, doing my testing uncovered another bug - rich text with newlines can result in multiple <br /> tags (see new Html test). The solution to this is a bit kludgey, but it does seem to fix both the Mpdf problem and the newly discovered Html problem. It will also tend to avoid the very minor problem of generating different line endings on Windows systems.

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

Why this change is needed?

Provide an explanation of why this change is needed, with links to any Issues (if appropriate).
If this is a bugfix or a new feature, and there are no existing Issues, then please also create an issue that will make it easier to track progress with this PR.