Add support for placeholders to Feign spring-mvc annotations of headers field

Hi, I saw there is an issue spring-cloud/spring-cloud-netflix#894 which add support for placeholders of value field.
But other field such as "headers" still not support placeholder.
Since sometimes we need set Authentication header, it is awesome if the value could be loaded from properties file.

@FeignClient("foo")
public interface FooClient{
    @RequestMapping(value = "${my.special.placeholder}",
            method = RequestMethod.GET,
            headers = "${my.token}"
    ResponseEntity getSomeData(s);
}

Start by tracing how Feign spring-mvc annotations currently resolve placeholders for the value field, then compare that path with handling of the headers field. Confirm the expected syntax from the issue example and verify that header placeholders load from application properties without breaking existing header behavior.