use seperate session in http downloader

gallery_dl/download.py

@@ -116,7 +116,7 @@ class DownloadJob():
         downloader = self.downloaders.get(scheme)
         if downloader is None:
             module = self.mngr.get_downloader_module(scheme)
-            downloader = module.Downloader(self.extractor)
+            downloader = module.Downloader()
             self.downloaders[scheme] = downloader
 
         return downloader

gallery_dl/downloader/http.py

@@ -4,9 +4,9 @@ import requests
 
 class Downloader(BasicDownloader):
 
-    def __init__(self, extr):
+    def __init__(self):
         BasicDownloader.__init__(self)
-        self.session = extr.session
+        self.session = requests.session()
 
     def download_impl(self, url, file):
         tries = 0

gallery_dl/downloader/text.py

@@ -2,7 +2,7 @@ from .common import BasicDownloader
 
 class Downloader(BasicDownloader):
 
-    def __init__(self, extr):
+    def __init__(self):
         BasicDownloader.__init__(self)
 
     def download_impl(self, url, file):