ContentFragment keep encoding data-caption

Description

When adding an attribute to a content fragment which also contains data-classid, the attribute's value keeps encoding.

Steps to reproduce:
1. Add block to tiny mce.

2. Inspect block and edit as html, add data-caption="Cat & Dog".

3. Publish the page (Note: data caption is encoded as expected).
4. Repeat steps 2 and 3 (Note: data caption is encoded again, and repeating these steps encodes them over and over).

Expected
Data-caption should be "Cat & Dog".

Actual

Data-caption is double encoded.

"Cat & Dog"