Update SignOptions to fix a type error

Author: lahirumaramba

test/resources/mocks.ts

@@ -205,8 +205,8 @@ export function generateIdToken(overrides?: object, claims?: object): string {
     expiresIn: ONE_HOUR_IN_SECONDS,
     issuer: 'https://securetoken.google.com/' + projectId,
     subject: uid,
-    algorithm: ALGORITHM,
     header: {
+      alg: ALGORITHM,
       kid: certificateObject.private_key_id,
     },
   }, overrides);
@@ -232,8 +232,8 @@ export function generateSessionCookie(overrides?: object, expiresIn?: number): s
     expiresIn: expiresIn || ONE_HOUR_IN_SECONDS,
     issuer: 'https://session.firebase.google.com/' + projectId,
     subject: uid,
-    algorithm: ALGORITHM,
     header: {
+      alg: ALGORITHM,
       kid: certificateObject.private_key_id,
     },
   }, overrides);
@@ -253,8 +253,8 @@ export function generateAppCheckToken(overrides?: object): string {
     expiresIn: ONE_HOUR_IN_SECONDS,
     issuer: 'https://firebaseappcheck.googleapis.com/' + projectNumber,
     subject: appId,
-    algorithm: ALGORITHM,
     header: {
+      alg: ALGORITHM,
       kid: jwksResponse.keys[0].kid,
     },
   }, overrides);