Xml Reader Rich Text

[oleibman]

Fix #4001. Thanks to @SlowFox71 who reported the problem and developed most of the solution. This PR adds Rich Text support to the XML reader. The Xml Spreadsheet stores Rich Text as Html tags, children of the ss:Data tag using a specific namespace. These can be parsed into a RichText object using existing method Helper/Html::toRichTextObject. There are 2 items which need special attention.

First, for attributes like bold or italic, Excel uses the appropriate Html tag (e.g. <B>). However, for an attribute like color, Excel uses <Font html:Color="#FF0000">, with a prefix on the Color tag. PhpSpreadsheet's Html parser cannot cope with the prefix. The parser is changed to strip html: from attribute names for the Font tag.

The example cited by the user used a <BR /> to indicate a line break in the data. However, it appears that, at least some of the time, Excel will instead use 
 to indicate a line break. The existing parser reduces one or more whitespace characters in the text to a single space, and so 
 will wind up disappearing. I am not sure why the existing code does this, but I do know that I am not willing to break it. Instead, I've added an optional boolean parameter $preserveWhiteSpace to toRichTextObject. If false (default), the existing logic will be used; but if true, substitution for whitespace characters in the text will not happen.

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
• x ] 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?

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