Suggest a fix if an object fails the be_instance_of matcher but its class...

...has the same name as the expected class

People commonly make the mistake of adding production classes to the spec target as well as the app target, which leads to confusing failures like: 

`Expected <<Foo: 0x7fa522ccf350> (Foo)> to be an instance of class <Foo>.`

This change tries to guide the developer to the most common solution for this issue. Perhaps we should consider adding a slightly more detailed guide to the wiki/FAQ with a couple Xcode screenshots pointing to the relevant Target Membership checkboxes and include a link to that in this suggestion string?

Hat tip to @xtreme-james-cooper and @pivotal-fahad-muntaz, the latest in a string of people who I have seen baffled by this issue recently.