Fix escape_once not working with hex entities or named entities endin…

[kainjow]

…g with numbers

Fixes #792

[fw42]

Thanks for your contribution! This makes sense to me.

@gauravmc, mind taking a quick look?

[kainjow]

I just noticed two issues actually:

• The 'x' for hex entities can be uppercased (just tested briefly with Safari/Chrome/Firefox)
• The hex characters is wrong, it should be stricter [\da-fA-F]

Let me go back and fix these. I'll open a new pull request.