Support hexBinary type in XML

[heka1024]

Motivation / Background

This Pull Request add hexBinary parser in XML. hexBinary is one of the primitive data type in XML https://www.w3.org/TR/xmlschema-2/#hexBinary. But rails does not support that type now.

Detail

This Pull Request add hex binary parser on ActiveSupport::XmlMini.

Additional information

• https://www.w3.org/TR/xmlschema-2/#hexBinary

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Unrelated changes should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.