fix(validation): update regex for basic auth

packages/fxa-auth-server/lib/oauth/validators.js

@@ -11,7 +11,7 @@ const config = require('../../config');
 
 exports.HEX_STRING = /^(?:[0-9a-f]{2})+$/;
 exports.B64URL_STRING = /^[A-Za-z0-9-_]+$/;
-exports.BASIC_AUTH_HEADER = /^Basic\s+([a-z0-9+\/]+)$/i;
+exports.BASIC_AUTH_HEADER = /^Basic\s+([a-zA-Z0-9+=\/]+)$/i;
 
 exports.clientId = Joi.string()
   .length(config.get('oauthServer.unique.id') * 2) // hex = bytes*2