update downloader.find() and related code

Instead of replacing 'https' with 'http' for every URL in
'get_downloader()', this now only happens once during downloader
initialization. Also unit tests.

test/test_downloader.py

@@ -8,13 +8,16 @@
 # published by the Free Software Foundation.
 
 import re
+import sys
 import base64
 import os.path
 import tempfile
-import unittest
 import threading
 import http.server
 
+import unittest
+from unittest.mock import Mock, MagicMock, patch
+
 import gallery_dl.downloader as downloader
 import gallery_dl.extractor as extractor
 import gallery_dl.config as config
@@ -23,6 +26,73 @@ from gallery_dl.output import NullOutput
 from gallery_dl.util import PathFormat
 
 
+class MockDownloaderModule(Mock):
+    __downloader__ = "mock"
+
+
+class TestDownloaderModule(unittest.TestCase):
+
+    @classmethod
+    def setUpClass(cls):
+        # allow import of ytdl downloader module without youtube_dl installed
+        sys.modules["youtube_dl"] = MagicMock()
+
+    @classmethod
+    def tearDownClass(cls):
+        del sys.modules["youtube_dl"]
+
+    def tearDown(self):
+        downloader._cache.clear()
+
+    def test_find(self):
+        cls = downloader.find("http")
+        self.assertEqual(cls.__name__, "HttpDownloader")
+        self.assertEqual(cls.scheme  , "http")
+
+        cls = downloader.find("https")
+        self.assertEqual(cls.__name__, "HttpDownloader")
+        self.assertEqual(cls.scheme  , "http")
+
+        cls = downloader.find("text")
+        self.assertEqual(cls.__name__, "TextDownloader")
+        self.assertEqual(cls.scheme  , "text")
+
+        cls = downloader.find("ytdl")
+        self.assertEqual(cls.__name__, "YoutubeDLDownloader")
+        self.assertEqual(cls.scheme  , "ytdl")
+
+        self.assertEqual(downloader.find("ftp"), None)
+        self.assertEqual(downloader.find("foo"), None)
+        self.assertEqual(downloader.find(1234) , None)
+        self.assertEqual(downloader.find(None) , None)
+
+    @patch("importlib.import_module")
+    def test_cache(self, import_module):
+        import_module.return_value = MockDownloaderModule()
+        downloader.find("http")
+        downloader.find("text")
+        downloader.find("ytdl")
+        self.assertEqual(import_module.call_count, 3)
+        downloader.find("http")
+        downloader.find("text")
+        downloader.find("ytdl")
+        self.assertEqual(import_module.call_count, 3)
+
+    @patch("importlib.import_module")
+    def test_cache_http(self, import_module):
+        import_module.return_value = MockDownloaderModule()
+        downloader.find("http")
+        downloader.find("https")
+        self.assertEqual(import_module.call_count, 1)
+
+    @patch("importlib.import_module")
+    def test_cache_https(self, import_module):
+        import_module.return_value = MockDownloaderModule()
+        downloader.find("https")
+        downloader.find("http")
+        self.assertEqual(import_module.call_count, 1)
+
+
 class TestDownloaderBase(unittest.TestCase):
 
     @classmethod