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