Update index.d.ts

The definition is missing an unset method found in superagent here https://github.com/visionmedia/superagent/blob/master/lib/request-base.js

superagent/index.d.ts

@@ -104,6 +104,7 @@ declare namespace request {
       set(field: Object): this;
       timeout(ms: number): this;
       type(val: string): this;
+      unset(field: string): this;
       use(fn: Function): this;
       withCredentials(): this;
       write(data: string, encoding?: string): this;