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/downloader/http.py

@@ -6,10 +6,9 @@
 # it under the terms of the GNU General Public License version 2 as
 # published by the Free Software Foundation.
 
-"""Downloader module for http:// and https:// urls"""
+"""Downloader module for http:// and https:// URLs"""
 
 import time
-import requests
 import requests.exceptions as rexcepts
 import mimetypes
 import logging
@@ -24,9 +23,9 @@ class Downloader(BasicDownloader):
     retries = config.interpolate(("downloader", "http", "retries",), 5)
     timeout = config.interpolate(("downloader", "http", "timeout",), None)
 
-    def __init__(self, output):
+    def __init__(self, session, output):
         BasicDownloader.__init__(self)
-        self.session = requests.session()
+        self.session = session
         self.out = output
 
     def download_impl(self, url, pathfmt):
@@ -96,17 +95,3 @@ class Downloader(BasicDownloader):
 
         # output for unrecoverable errors
         self.out.error(pathfmt.path, msg, tries, 0)
-
-    def set_headers(self, headers):
-        """Set headers for http requests"""
-        self.set_dict(self.session.headers, headers)
-
-    def set_cookies(self, cookies):
-        """Set cookies for http requests"""
-        self.set_dict(self.session.cookies, cookies)
-
-    @staticmethod
-    def set_dict(dest, src):
-        """Copy the contents of dictionary 'src' to 'dest'"""
-        dest.clear()
-        dest.update(src)