Accept Incorrectly Capitalized Content-Range Headers

Some external podcast urls return a valid CDN response, which is not accepted due to the "Content-Range" header being all lowercase as "content-range". This tries the lowercase version of the header as a backup if no Content-Range header is found initially.

librespot/audio/__init__.py

@@ -578,6 +578,8 @@ class CdnManager:
             response = self.request(range_start=0,
                                     range_end=ChannelManager.chunk_size - 1)
             content_range = response.headers.get("Content-Range")
+            if content_range is None:
+                content_range = response.headers.get("content-range")
             if content_range is None:
                 raise IOError("Missing Content-Range header!")
             split = content_range.split("/")