[Generator] Improve parameter handling of MIME types in content types (…

…#646)

### Motivation

The generator piece of
apple/swift-openapi-runtime#113.

This is needed for APIs such as Kubernetes, that has both
`application/json` and `application/json; watch=true` in a single
operation.

Currently the parameters weren't taken into account, which could lead to
incorrect matching of content types.

### Modifications

Use the full MIME type, including the parameters, for matching.

### Result

More correct behavior, now we correctly handle multiple MIME types with
the same type/subtype in an operation.

### Test Plan

Added a unit test.

Sources/_OpenAPIGeneratorCore/Translator/Content/ContentType.swift

@@ -194,7 +194,7 @@ struct ContentType: Hashable {
     /// The header value used when validating a content-type header.
     ///
     /// This should be less strict, e.g. not require `charset`.
-    var headerValueForValidation: String { lowercasedTypeAndSubtype }
+    var headerValueForValidation: String { lowercasedTypeSubtypeAndParameters }
 
     /// The coding strategy appropriate for this content type.
     var codingStrategy: CodingStrategy { category.codingStrategy }