Fix undefined method readpartial' for \"\":String` error.

This exception was thrown by app/logical/pixiv_ajax_client.rb:406 when a
Pixiv API call failed with a network error. In this case we tried to log
the response body, but this failed because we returned a faked HTTP
response with an empty string for the body, which the http.rb library
didn't like because it was expecting an IO-like object for the body.

test/unit/danbooru_http_test.rb

@@ -22,11 +22,13 @@ class DanbooruHttpTest < ActiveSupport::TestCase
       should "fail if redirected too many times" do
         response = Danbooru::Http.get(httpbin_url("absolute-redirect/10"))
         assert_equal(596, response.status)
+        assert_equal("", response.body.to_s)
       end
 
       should "fail if the request takes too long to connect" do
         response = Danbooru::Http.timeout(1).get(httpbin_url("delay/5"))
         assert_equal(597, response.status)
+        assert_equal("", response.body.to_s)
       end
 
       should "fail if the request takes too long to download" do
@@ -39,11 +41,13 @@ class DanbooruHttpTest < ActiveSupport::TestCase
       should "return a 5xx error if the domain can't be resolved" do
         response = Danbooru::Http.get("http://doesnotexist.donmai.us")
         assert_equal(598, response.status)
+        assert_equal("", response.body.to_s)
       end
 
       should "return a 5xx error if the SSL certificate is expired" do
         response = Danbooru::Http.get("https://expired.badssl.com")
         assert_equal(590, response.status)
+        assert_equal("", response.body.to_s)
       end
 
       should "automatically decompress gzipped responses" do