Only verify download size in requests downloader if possible

Some Servers may not response with the Content-Length header, even if it's from segmented media. I.e., if it's a subtitle URL. The requests downloader required the header to be present as it downloads each URL, which is not possible.

Now it tries to get it if possible, and verifies the download size with the Content-Length value if it could be obtained.

devine/core/downloaders/requests.py

@@ -57,7 +57,7 @@ def requests(
     for url, out_path in uri:
         out_path.parent.mkdir(parents=True, exist_ok=True)
         stream = session.get(url, stream=True)
-        file_size = int(stream.headers["Content-Length"])
+        file_size = stream.headers.get("Content-Length")
         with open(out_path, "wb") as f:
             written = 0
             for chunk in stream.iter_content(chunk_size=1024):
@@ -77,7 +77,7 @@ def requests(
                         progress(downloaded=f"{filesize.decimal(download_speed)}/s")
                         last_speed_refresh = now
                         download_sizes.clear()
-        if written < file_size:
+        if file_size and written < int(file_size):
             raise ValueError(
                 f"{url} finished downloading unexpectedly, got {decimal(written)}/{decimal(file_size)}")