Bug 980478 - Pre: Update names.
Bug 980478 - Pre: Test against stage token server.
Bug 980478 - Part 1: Re-order parameters to getPayloadString to match consumers.
Bug 980478 - Part 2: Prefer expiresAt in JSONWebTokenUtils.createAssertion.
Bug 980478 - Part 3: Prefer expiresAt to duration in MockMyIDTokenFactory.createMockMyIDCertificate.
Bug 980478 - Part 4: Prefer expiresAt to duration in MockMyIDTokenFactory.createMockMyIDAssertion.
Bug 980478 - Part 5: Simplify JSONWebTokenUtils.
Bug 980478 - Part 6: Allow null issuedAt.
Bug 980478 - Part 7: Generate assertions with no iat and exp: 9999999999999L.
Bug 980478 - Generate assertions with no iat and exp: 9999999999999L. r=rnewman