Fix case-insensitive comparison in get_adapter()

While trying to get the prefix for an url, the url was lowered before comparing but the prefix not, so if it contains non-lowercase characters (eg. https://api.example.com/sOmE_WeiRD_pReFIX/), it won't match.

requests/sessions.py

@@ -696,7 +696,7 @@ class Session(SessionRedirectMixin):
         """
         for (prefix, adapter) in self.adapters.items():
 
-            if url.lower().startswith(prefix):
+            if url.lower().startswith(prefix.lower()):
                 return adapter
 
         # Nothing matches :-/