Fix httpx oauth1 binary form data

[shc261392]

See issue https://github.com/authlib/authlib/issues/516 for details.

What kind of change does this PR introduce? (check at least one)

• [v] Bugfix
• [ ] Feature
• [ ] Code style update
• [ ] Refactor
• [ ] Other, please describe:

Does this PR introduce a breaking change? (check one)

• [ ] Yes
• [v] No

---

• [v] You consent that the copyright of your pull request source code belongs to Authlib's author.

[sonarqubecloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud