fix encoding for proxy auth token

Signed-off-by: Brian DeHamer <bdehamer@github.com>

src/index.ts

@@ -726,7 +726,9 @@ export class HttpClient {
       uri: proxyUrl.href,
       pipelining: !this._keepAlive ? 0 : 1,
       ...((proxyUrl.username || proxyUrl.password) && {
-        token: `${proxyUrl.username}:${proxyUrl.password}`
+        token: `Basic ${Buffer.from(
+          `${proxyUrl.username}:${proxyUrl.password}`
+        ).toString('base64')}`
       })
     })
     this._proxyAgentDispatcher = proxyAgent