Increase test coverage for low coverage classes

[runarmyklebust]

Hypothesis; there are a lot of classes that with low test coverage that is easily tested with a minor effort. Identify and create tasks.

[alansemenov]

@aro please make the list of classes that are missing tests, either entirely or for the most part

[aro]

Test classes in server-rest module. Resource classes in package com.enonic.xp.impl.server.rest:

• ApplicationResource
• ContentResource
• ExportResource
• IndexResource
• SnapshotResource
• SystemResource