nicoseiga: optimize image_url method.

The image_url method makes a request to `https://seiga.nicovideo.jp/images/source/:image_id`
to see where this URL redirects to. Before we did a GET request, which caused it to download
the full image. This could fail with a timeout error if the download took too long. We also
cached the request, which caused the full image to be cached, even though we only need the
headers. Change it to a HEAD request so we don't have to download the entire image just to
check the URL.

app/logical/nico_seiga_api_client.rb

@@ -78,7 +78,7 @@ class NicoSeigaApiClient
     Hash.from_xml(resp.to_s)["response"]["user"]
   end
 
-  def get(url)
+  def login
     form = {
       mail_tel: Danbooru.config.nico_seiga_login,
       password: Danbooru.config.nico_seiga_password
@@ -88,7 +88,11 @@ class NicoSeigaApiClient
     resp = http.cache(1.hour).post("https://account.nicovideo.jp/login/redirector?site=seiga", form: form)
     raise RuntimeError, "NicoSeiga login failed (status=#{resp.status} url=#{url})" if resp.status != 200
 
-    resp = http.cache(1.minute).get(url)
+    http
+  end
+
+  def get(url)
+    resp = login.cache(1.minute).get(url)
     #raise RuntimeError, "NicoSeiga get failed (status=#{resp.status} url=#{url})" if resp.status != 200
 
     resp