Explicitly add stream to cache

Pacote doesn't do this automatically anymore

Closes npm/pacote#73 & Closes npm#2160

lib/cache.js

@@ -105,12 +105,11 @@ with --force.`)
 
     log.silly('cache add', 'spec', spec)
 
-    // we ask pacote for the thing, and then just throw the data
-    // away so that it tee-pipes it into the cache like it does
-    // for a normal request.
     await pacote.tarball.stream(spec, stream => {
       stream.resume()
-      return stream.promise()
+      return cacache.put.stream(this.npm.config.get('cache'), stream)
+        .pipe(stream).promise()
+      })
     }, this.npm.flatOptions)
   }