deps: update quick-xml dependency

[decathorpe]

The quick-xml dependency is currently pinned to v0.25, while upstream has already released v0.29.
However, I'm not sure how to handle some of the breaking changes since v0.25:

1. The $text attribute (added in v0.28.0) breaks the new custom Value deserializer (though adding a case for "$text" to the match statement seems to solve that (not sure if that's correct, but it seems to work 😕)
2. quick-xml now defaults to writing self-closing XML tags when serializing empty strings (i.e. empty string values are written as <value><string /></value> and empty bytes are written as <value><base64 /></value>. While this is valid XML and should be acceptable for XML-RPC, it's inconsistent with other implementations

I've checked how Python's xmlrpc module handles this (see below) - it supports reading XML with self-closing tags, but it does not seem to ever write them. I don't think there's a way to control this behaviour from the quick-xml serializer side (without writing a custom serializer ...)

---

import xmlrpc.client
xmlrpc.client.loads("<params>\n<param>\n<value><string /></value>\n</param>\n</params>\n")
# (('',), None)
xmlrpc.client.loads("<params>\n<param>\n<value><string></string></value>\n</param>\n</params>\n")
# (('',), None)
xmlrpc.client.dumps(("",))
# '<params>\n<param>\n<value><string></string></value>\n</param>\n</params>\n'

[decathorpe]

@bahlo You seem to have more experience with weird XML-RPC clients / servers than me - do you think it would be an issue if dxr clients / servers started sending XML-RPC messages that contained self-closing XML tags for empty strings / bytes?

[decathorpe]

Correction: quick-xml seems to write <string/> and <base64/> without a space before />. It looks weird to me for some reason, but it appears to be valid XML 1.0 syntax.

---

EDIT: 407a3a1 is how far I've gotten. I needed to adapt some tests for the behaviour change, but everything else still seems to work.

[bahlo]

Hah yeah I sadly do—the server I‘m interfacing with is using XMLRPC++ 0.7 and does not support self-closing xml tags so I‘d selfishly advise to not use that feature 🥸

[decathorpe]

For some reason I suspected as much 😓
I opened an issue with quick-xml upstream, maybe this can be worked around somehow:
tafia/quick-xml#617

[decathorpe]

quick-xml v0.30 was released with the new API.

This commit updates the dependency and adjusts the code accordingly:
ee87998

Will be part of the upcoming (soon!) v0.6.0 release.

[bahlo]

appreciate it!