Remove extraneous objects if updatecheck status = noupdate

[mschfh]

The current implementation returns manifest and urls objects even if there is no update available:

go-update/server/server_test.go

Lines 184 to 197 in 96c047b

	expectedResponse = `<response protocol="3.1" server="prod">
	<app appid="ldimlcelhnjgpjjemdjokpgeeikdinbm">
	<updatecheck status="noupdate">
	<urls>
	<url codebase="https://` + extension.GetS3ExtensionBucketHost(lightThemeExtensionID) + `/release/ldimlcelhnjgpjjemdjokpgeeikdinbm/extension_1_0_0.crx"></url>
	</urls>
	<manifest version="1.0.0">
	<packages>
	<package name="extension_1_0_0.crx" hash_sha256="1c714fadd4208c63f74b707e4c12b81b3ad0153c37de1348fa810dd47cfc5618" required="true"></package>
	</packages>
	</manifest>
	</updatecheck>
	</app>
	</response>`

According to the spec, those should not be set unless there is an update available:

Additionally, the following members are set if and only if status == "ok":
manifest: A manifest object.
urls: A urls object.