Update index.js

Fix wrong comparison.
this.textEncoding was set before, this.transferEncoding is always undefined

lib/mime-node/index.js

@@ -451,7 +451,7 @@ class MimeNode {
                         transferEncoding = this._getTextEncoding(this.content) === 'Q' ? 'quoted-printable' : 'base64';
                     } else {
                         // we can not check content for a stream, so either use preferred encoding or fallback to QP
-                        transferEncoding = this.transferEncoding === 'B' ? 'base64' : 'quoted-printable';
+                        transferEncoding = this.textEncoding === 'B' ? 'base64' : 'quoted-printable';
                     }
                 } else if (!/^(multipart|message)\//i.test(contentType)) {
                     transferEncoding = transferEncoding || 'base64';