Fix code scanning alert no. 7: Cross-site scripting

[velo]

Fixes https://github.com/OpenFeign/feign/security/code-scanning/7

To fix the problem, we need to ensure that the user-provided content type is properly sanitized or encoded before being included in the response. The best way to fix this issue is to use a library that provides HTML encoding to prevent XSS attacks. In Java, the StringEscapeUtils class from the Apache Commons Text library can be used for this purpose.

• General Fix: Encode the user-provided content type before including it in the response body.
• Detailed Fix: Modify the uploadUnknownType method to encode the content type using StringEscapeUtils.escapeHtml4.
• Files/Regions/Lines to Change: Modify lines 161-164 in feign-form/src/test/java/feign/form/Server.java.
• Needed Imports: Add an import statement for org.apache.commons.text.StringEscapeUtils.

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