comments: allow swapping votes.

Allow users to upvote a comment, then downvote it, without raising an
error or having to manually remove the upvote first. The upvote is
automatically removed and replaced by the downvote.

Changes to the /comment_votes API:

* `POST /comment_votes` and `DELETE /comment_votes` now return a comment
  vote instead of a comment.
* The `score` param in `POST /comment_votes` is now 1 or -1, not
  `up` or `down.`

test/unit/comment_vote_test.rb

@@ -0,0 +1,57 @@
+require 'test_helper'
+
+class CommentVoteTest < ActiveSupport::TestCase
+  context "A CommentVote" do
+    setup do
+      @user = create(:user)
+      @comment = as(@user) { create(:comment) }
+    end
+
+    context "during validation" do
+      subject { build(:comment_vote, comment: as(@user) { create(:comment) }) }
+
+      should validate_uniqueness_of(:user_id).scoped_to(:comment_id).with_message("have already voted for this comment")
+      should validate_inclusion_of(:score).in_array([-1, 1]).with_message("must be 1 or -1")
+    end
+
+    context "creating" do
+      context "an upvote" do
+        should "increment the comment's score" do
+          vote = create(:comment_vote, comment: @comment, score: 1)
+
+          assert_equal(1, @comment.reload.score)
+        end
+      end
+
+      context "a downvote" do
+        should "decrement the comment's score" do
+          vote = create(:comment_vote, comment: @comment, score: -1)
+
+          assert_equal(-1, @comment.reload.score)
+        end
+      end
+    end
+
+    context "destroying" do
+      context "an upvote" do
+        should "decrement the comment's score" do
+          vote = create(:comment_vote, comment: @comment, score: 1)
+          assert_equal(1, @comment.reload.score)
+
+          vote.destroy
+          assert_equal(0, @comment.reload.score)
+        end
+      end
+
+      context "a downvote" do
+        should "increment the comment's score" do
+          vote = create(:comment_vote, comment: @comment, score: -1)
+          assert_equal(-1, @comment.reload.score)
+
+          vote.destroy
+          assert_equal(0, @comment.reload.score)
+        end
+      end
+    end
+  end
+end