uploads: fix "ArgumentError: string contains null byte" error

Fix an error when trying to upload a file larger than the file size
limit. In this case we tried to dump the whole HTTP response into the
error message, which included the binary file itself, which caused this
exception because it contained null bytes.

test/functional/uploads_controller_test.rb

@@ -177,18 +177,32 @@ class UploadsControllerTest < ActionDispatch::IntegrationTest
         assert_match("Not an image or video", Upload.last.error)
       end
 
-      should "fail if the file size is too large" do
-        skip "flaky test"
-        Danbooru.config.stubs(:max_file_size).returns(1.kilobyte)
+      context "for a file larger than the file size limit" do
+        setup do
+          skip "flaky test"
+          Danbooru.config.stubs(:max_file_size).returns(1.kilobyte)
+        end
 
-        file = Rack::Test::UploadedFile.new("test/files/test.jpg")
-        post_auth uploads_path(format: :json), @user, params: { upload: { file: file }}
-        perform_enqueued_jobs
+        should "fail for a direct file upload" do
+          create_upload!("test/files/test.jpg", user: @user)
 
-        assert_response 201
-        assert_match("File size must be less than or equal to", Upload.last.error)
+          assert_response 201
+          assert_match("File size too large", Upload.last.error)
+        end
 
-        Danbooru.config.unstub(:max_file_size)
+        should "fail for a source upload with a Content-Length header" do
+          create_upload!("https://nghttp2.org/httpbin/bytes/2000", user: @user)
+
+          assert_response 201
+          assert_match("File size too large", Upload.last.error)
+        end
+
+        should "fail for a source upload without a Content-Length header" do
+          create_upload!("https://nghttp2.org/httpbin/stream-bytes/2000", user: @user)
+
+          assert_response 201
+          assert_match("File size too large", Upload.last.error)
+        end
       end
 
       context "for a corrupted image" do