add accept header select to async method

Author: wing328

modules/swagger-codegen/src/main/resources/csharp/api.mustache

@@ -166,7 +166,17 @@ namespace {{packageName}}.Api
             var formParams = new Dictionary<String, String>();
             var fileParams = new Dictionary<String, FileParameter>();
             String postBody = null;
-    
+
+            // to determine the Accept header
+            String[] http_header_accepts = new String[] {
+                {{#produces}}"{{mediaType}}"{{#hasMore}}, {{/hasMore}}{{/produces}}
+            };
+            String http_header_accept = ApiClient.SelectHeaderAccept(http_header_accepts);
+            if (http_header_accept != null)
+                headerParams.Add("Accept", ApiClient.SelectHeaderAccept(http_header_accepts));
+
+            // set "format" to json by default
+            // e.g. /pet/{petId}.{format} becomes /pet/{petId}.json
             pathParams.Add("format", "json");
             {{#pathParams}}if ({{paramName}} != null) pathParams.Add("{{baseName}}", ApiClient.ParameterToString({{paramName}})); // path parameter
             {{/pathParams}}