[downloader/http] Only check for resumability when actually resuming

test/test_downloader_http.py

@@ -88,7 +88,7 @@ class TestHttpFD(unittest.TestCase):
         self.assertTrue(downloader.real_download(filename, {
             'url': 'http://127.0.0.1:%d/%s' % (self.port, ep),
         }))
-        self.assertEqual(os.path.getsize(encodeFilename(filename)), TEST_SIZE)
+        self.assertEqual(os.path.getsize(encodeFilename(filename)), TEST_SIZE, ep)
         try_rm(encodeFilename(filename))
 
     def download_all(self, params):

youtube_dl/downloader/http.py

@@ -141,6 +141,7 @@ class HttpFD(FileDownloader):
                     # Content-Range is either not present or invalid. Assuming remote webserver is
                     # trying to send the whole file, resume is not possible, so wiping the local file
                     # and performing entire redownload
+                    if range_start > 0:
                         self.report_unable_to_resume()
                     ctx.resume_len = 0
                     ctx.open_mode = 'wb'