superagent: Fix callback signature

Error can be an error object or null at least (and possibly some other
values, like undefined, though those aren’t documented). There was
already a test with error being null, but it was passing only because
strictNullChecks were off. Now that strictNullChecks are on, this must
be changed.

superagent/index.d.ts

@@ -49,7 +49,7 @@ declare namespace request {
         search(url: string, callback?: CallbackHandler): Req;
         connect(url: string, callback?: CallbackHandler): Req;
 
-      parse(fn: (res: Response, callback: (err: Error, body: any) => void) => void): this;
+      parse(fn: (res: Response, callback: (err: Error | null, body: any) => void) => void): this;
       saveCookies(res: Response): void;
       attachCookies(req: Req): void;
     }
@@ -109,7 +109,7 @@ declare namespace request {
       withCredentials(): this;
       write(data: string, encoding?: string): this;
       write(data: Buffer, encoding?: string): this;
-      parse(fn: (res: Response, callback: (err: Error, body: any) => void) => void): this;
+      parse(fn: (res: Response, callback: (err: Error | null, body: any) => void) => void): this;
     }
 
 }

superagent/superagent-tests.ts

@@ -303,7 +303,7 @@ request
 request
   .get('/search')
   .then((response) => {})
-  .catch((error) => {}); 
+  .catch((error) => {});
 // Requesting binary data.
 // adapted from: https://github.com/visionmedia/superagent/blob/v2.0.0/test/client/request.js#L110
 request