uploads: fix not being able to change the source field during upload.

Fix not being able to change the post's source when submitting the
upload. For example, if you were uploading a Twitter image from a direct
Twitter image URL, and you tried to change the source to the tweet URL
on the upload page before creating the post, then the source would be
ignored when the post was created.

test/functional/posts_controller_test.rb

@@ -663,6 +663,13 @@ class PostsControllerTest < ActionDispatch::IntegrationTest
         assert_equal("tagme", @post.tag_string)
       end
 
+      should "set the source" do
+        @post = create_post!(source: "https://www.example.com")
+
+        assert_redirected_to @post
+        assert_equal("https://www.example.com", @post.source)
+      end
+
       should "autoban the post when it is tagged banned_artist" do
         @post = create_post!(tag_string: "banned_artist")
         assert_equal(true, @post.is_banned?)