Move and refactor method from follow to user model to get user interests. Add specification to check the discard of duplicate interests.

spec/models/follow_spec.rb

@@ -23,17 +23,4 @@ describe Follow do
     expect(follow).to_not be_valid
   end
 
-  describe "interests" do
-
-    let(:user)     { create(:user) }
-
-    it "interests_by user" do
-      proposal =  create(:proposal, tag_list: "Sport")
-      create(:follow, :followed_proposal, followable: proposal, user: user)
-
-      expect(Follow.interests_by(user)).to eq ["Sport"]
-    end
-
-  end
-
 end