Warn on formdata parameters with no matching content type

This is a joint issue with https://github.com/swagger-api/swagger-codegen/issues/4853

formData parameters require a Content-Type set to multipart/form-data or application/x-www-form-urlencoded. The client generator assumes that application/json is a reasonable default when the API doesn't contain a consumes directive.

This means that it's easy to write an API that simply doesn't work.

The parser does not emit an error in this case. Neither does the code generator.

Since there are two options with different tradeoffs for the content type, it may not be appropriate for the code generator to intuit an appropriate content type. Since this is a facet of HTTP instead of a specific codegen target, it's probably appropriate for the parser to emit a warning here.

Start by tracing the parser's handling of formData parameters and the consumes directive; the issue does not name specific files or tests. Confirm how missing or incompatible content types are currently handled, then add coverage showing that the parser emits a warning without choosing a content type.