Update type to fix tests (#620)

Author: kevinajian

spec/fixtures/mockrequest.ts

@@ -68,7 +68,7 @@ export function mockFetchPublicKeys(): nock.Scope {
  */
 export function generateIdToken(projectId: string): string {
   const claims = {};
-  const options = {
+  const options: jwt.SignOptions = {
     audience: projectId,
     expiresIn: 60 * 60, // 1 hour in seconds
     issuer: 'https://securetoken.google.com/' + projectId,