Danbooru::Http: redirect POST to GET on 302.

When a POST request returns a 302 redirect, follow the redirect with a
GET request instead of with a POST request.

HTTP standards leave it unspecified whether a POST request that returns
a 302 redirect should be followed with a GET or with a POST. A GET is
what most browsers use, which means it's what most servers expect.

Fixes the /tagme Discord command not working because when we uploaded
the image to DeepDanbooru, the POST request returned a 302 redirect,
which the server expected us to follow with a GET, not with a POST.

Ref:

* https://stackoverflow.com/questions/17605915/what-is-the-correct-behavior-expected-of-an-http-post-302-redirect-to-get

app/logical/danbooru/http/redirector.rb

@@ -31,7 +31,7 @@ module Danbooru
         uri = HTTP::URI.parse(location)
 
         verb = request.verb
-        verb = :get if response.status == 303 && !request.verb.in?([:get, :head])
+        verb = :get if response.status.in?([302, 303]) && !request.verb.in?([:get, :head])
 
         request.redirect(uri, verb)
       end

test/unit/danbooru_http_test.rb

@@ -78,6 +78,13 @@ class DanbooruHttpTest < ActiveSupport::TestCase
       end
     end
 
+    context "#post method" do
+      should "follow 302 redirects with a GET" do
+        response = Danbooru::Http.get(httpbin_url("redirect-to?url=#{httpbin_url("get")}"))
+        assert_equal(200, response.status)
+      end
+    end
+
     context "cache feature" do
       should "cache multiple requests to the same url" do
         http = Danbooru::Http.cache(1.hour)