Make Content-Type validation robust to MIME parameters and case-insensitive per RFC 9110

[tomdesair]

Summary

• Add Utils.isMediaType(String contentTypeHeader, String expectedMediaType) and Utils.extractMediaType(String contentTypeHeader) to parse HTTP media types per RFC 9110 §8.3 (stripping parameters such as ;charset=UTF-8 and performing case-insensitive matching).
• Define HttpHeader.CONTENT_TYPE_OFFSET_OCTET_STREAM constant.
• Update ContentTypeValidator, PostContentTypeValidator, RufhAppendValidator, and protocol version autodetection in Utils to use Utils.isMediaType(...) and the HttpHeader constant.
• Add unit tests covering charset parameters, whitespace variations, and mixed casing across UtilsTest, ContentTypeValidatorTest, and PostContentTypeValidatorTest (100% diff code coverage).

[coveralls]

coverage: 94.62% (+0.04%) from 94.583% — bugfix/robust-content-type-validation into master