Modify XLSX RW to keep decimal for floats with a zero decimal part

[rtek]

This is:

• a bugfix

Why this change is needed?

Prior to 1.10, all numeric values where read as floats. In 1.10
numeric values are read using 0 + x, which relies on PHP type
juggling rules. As a result, float(0.0) is written as string('0'),
then read back as int(0). This fix causes the writer to retain the
the decimal for float values such that a reader can differentiate
floats from ints.

Checklist:

• Changes are covered by unit tests
• 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
• Documentation is updated as necessary

[PowerKiKi]

Thanks for this great PR !