Filter URLs that aren't strings
Validate ID on input
Validate that the type of an activity object is a string
It's OK to have no ID
Validate that attachments is an array
Check that attachments only includes objects
Validate that attachments array is valid objects
Better toString() for activity objects and more validation
Validate author property is a real activity object
Content has to be a string
Validate that displayName is a string
Add a comment
Check that downstreamDuplicates is an array
Check that downstreamDuplicates is all strings
Image proprety must be an object
Return from failure
Validate the image property
test that image.url is a string
Validate image.width
Validate image.height