#567 concluded that more checks should be done upfront in the Contract. This PR aims to perform both the Map validation AND the Map's String key validation in the contract. This reduces the number of checks that happen at "runtime" within the ReflectiveFeign class

This change breaks clients which use a method parameter as HeaderMap which is non-generic (like Spring's HttpHeaders).

The following used to work before the PR (when using spring-cloud-openfeign):

@RequestMapping(method = RequestMethod.POST, path = "/path")
void method(@RequestHeader HttpHeaders headers);

It now fails with java.lang.ClassCastException: java.lang.Class cannot be cast to java.lang.reflect.ParameterizedType as HttpHeaders is non-generic but implements MultiValueMap<String, String> which in turn implements Map<String, List<String>>.

As a workaround HttpHeaders can be changed to MultiValueMap<String, List<String>>.

@sunruh this bug was corrected in Feign 9.7 via #689

I must have missed that PR looking for fixes. Thanks for the quick feedback.