dl_files: use HEAD when no content is desired.

This function requests the file index of the specified database, to
check whether it exists, but does not actually read its contents.
This is a waste of bandwidth and a waste of processing time on both
ends.  Change this to perform a HEAD request instead.

Author: Benjamin Moody

wfdb/io/download.py

@@ -511,7 +511,7 @@ def dl_files(db, dl_dir, files, keep_subdirs=True, overwrite=False):
     db_url = posixpath.join(PN_CONTENT_URL, db_dir) + '/'
 
     # Check if the database is valid
-    response = requests.get(db_url)
+    response = requests.head(db_url)
     response.raise_for_status()
 
     # Construct the urls to download