Use first Content-length header (bug 1226)

Use first matching Content-length header when multiple
found. Occured with some cache/proxy servers.

src/gpodder/download.py

@@ -297,7 +297,7 @@ def retrieve_resume(self, url, filename, reporthook=None, data=None):
         blocknum = int(current_size/bs)
         if reporthook:
             if "content-length" in headers:
-                size = int(headers["Content-Length"]) + current_size
+                size = int(headers.getrawheader("Content-Length"))  + current_size
             reporthook(blocknum, bs, size)
         while read < size or size == -1:
             if size == -1: