share extractor and downloader sessions

There was never any "good" reason for the strict separation
between extractors and downloaders. This change allows for
reduced resource usage (probably unnoticeable) and less lines
of code at the "cost" of tighter coupling.

gallery_dl/extractor/exhentai.py

@@ -50,9 +50,8 @@ class ExhentaiGalleryExtractor(Extractor):
 
     def items(self):
         self.login()
+        self.setup_headers()
         yield Message.Version, 1
-        yield Message.Headers, self.setup_headers()
-        yield Message.Cookies, self.session.cookies
 
         url = "{}/g/{}/{}/".format(self.root, self.gid, self.token)
         response = self.session.get(url)
@@ -76,14 +75,9 @@ class ExhentaiGalleryExtractor(Extractor):
         """Initialize headers"""
         self.session.headers.update({
             "User-Agent": "Mozilla/5.0",
-            "Accept": "text/html,application/xhtml+xml,"
-                      "application/xml;q=0.9,*/*;q=0.8",
             "Accept-Language": "en-US,en;q=0.5",
             "Referer": self.root + "/",
         })
-        headers = self.session.headers.copy()
-        headers["Accept"] = "image/png,image/*;q=0.8,*/*;q=0.5"
-        return headers
 
     def get_job_metadata(self, page):
         """Collect metadata for extractor-job"""