[superagent] Fix Blob error

At the moment, some users' builds fail, because `Blob` isn't defined as
a global in Node.js - it sits in the [`buffer`][1] package.

This change adds an import from `buffer` to fix this issue.

Note that I think the build was incorrectly passing before because of
a [leaky interface in `stream/consumers`][2].

[1]: https://nodejs.org/api/buffer.html#class-blob
[2]: DefinitelyTyped#55311 (comment)

types/superagent/index.d.ts

@@ -20,6 +20,7 @@ import * as fs from "fs";
 import * as http from "http";
 import * as stream from "stream";
 import * as cookiejar from "cookiejar";
+import { Blob } from 'buffer';
 
 type CallbackHandler = (err: any, res: request.Response) => void;
 

types/superagent/superagent-tests.ts

@@ -3,6 +3,7 @@ import request = require("superagent");
 import * as fs from "fs";
 import assert = require("assert");
 import { Agent } from "https";
+import { Blob } from "buffer";
 
 // Examples taken from https://github.com/visionmedia/superagent/blob/gh-pages/docs/index.md
 // and https://github.com/visionmedia/superagent/blob/master/Readme.md
@@ -219,7 +220,7 @@ request.get("http://example.com/search").retry(2, callback).end(callback);
 })();
 
 // Attaching files
-declare const blob: Blob;
+const blob = new Blob([]);
 request
     .post("/upload")
     .attach("avatar", "path/to/tobi.png", "user.png")