HeaderMap()
Adds headers specified in the Map or okhttp3.Headers.
Values in the map are converted to strings using Retrofit#stringConverter(Type, * Annotation[]) (or Object#toString(), if no matching string converter is installed).
Simple Example:
@GET("/search")
void list(@HeaderMap Map<String, String> headers);
...
// The following call yields /search with headers
// Accept: text/plain and Accept-Charset: utf-8
foo.list(ImmutableMap.of("Accept", "text/plain", "Accept-Charset", "utf-8"));