Fix templated Content-Type handling in SpringEncoder

[yashdotdev13]

Description

Fixes #116

SpringEncoder previously attempted to parse an unresolved templated Content-Type value, such as {Content-Type}, using MediaType.valueOf(...), which resulted in an invalid media type exception.

This change handles unresolved Content-Type templates by skipping media type parsing and removing the unresolved template before creating the HttpHeaders passed to Spring's message converters.

Changes

• Skip MediaType parsing for unresolved Content-Type templates.
• Remove unresolved Content-Type templates from headers passed to message converters.
• Add a regression test for encoding requests with a templated Content-Type header.

Testing

• Added a regression test in SpringEncoderTests.
• Verified the targeted unit and integration tests.
• Ran the full build successfully:

mvn clean verify

[kdelay]

On 86bda63, JDK 17: core module 426 tests / 0 failures. Two gaps before this closes #116:

1. The template is dropped, not deferred: the headers get overwritten with the converter's, so {Content-Type} plus a POJO body ends up application/json, and resolve(Content-Type=application/xml) still gives application/json. Re-applying the saved template after request.headers(newHeaders) makes resolve give application/xml, suite still 426/0.

2. application/{ct} still throws InvalidMediaTypeException at SpringEncoder:99.

The new test asserts only body() != null, so it passes under (1).