Check exact array contents instead of inclusion

We're using `eq` and `match_array` in most places, but there were a few
places where we were still checking each element is included in the
array. This is a bit dangerous, because the array could have duplicate
elements, and we wouldn't detect them with `include`.

spec/lib/email_digests_spec.rb

@@ -14,7 +14,7 @@ describe EmailDigest do
 
       email_digest = EmailDigest.new(user1)
 
-      expect(email_digest.notifications).to include(notification1)
+      expect(email_digest.notifications).to eq [notification1]
       expect(email_digest.notifications).not_to include(notification2)
     end
 
@@ -29,7 +29,7 @@ describe EmailDigest do
 
       email_digest = EmailDigest.new(user)
 
-      expect(email_digest.notifications).to include(notification1)
+      expect(email_digest.notifications).to eq [notification1]
       expect(email_digest.notifications).not_to include(notification2)
     end