Entity Reference handling (when importing from XML)

I noticed that Entity Referenced characters (< > & ' ") are not handled properly. Only decimal format is supported, hexadecimal and character forms are not supported yet.

---
##### Current status

| Format | Supported |
| --- | --- |
| & (decimal form) | ✓ |
| & (hexadecimal form) | ✗ |
| & (character form) | ✗ |

---
##### Example of the issue
1. Source XML

> <string> &amp; </string>
2. Imported as

> &amp;
3. Exported XML

> <string> &amp;amp; </string>

---
##### References
- [List of XML and HTML character entity references - Wikipedia, the free encyclopedia](http://en.wikipedia.org/wiki/List_of_XML_and_HTML_character_entity_references)

Start by tracing the XML import path in this PHP repository and compare its handling of decimal, hexadecimal, and character entity forms. Check the existing import behavior against the issue's source, imported, and exported XML examples. Done means all listed forms are imported without becoming double-escaped on export, with coverage for each form.

Written by the indexing model from the issue text.