_header_x now defaults to empty string

Prior to this a request to e.g. https://latest.datasette.io/fixtures/magic_parameters
which did not include a User-Agent header would trigger a 500 error.

datasette/default_magic_parameters.py

@@ -8,7 +8,7 @@ import time
 def header(key, request):
     key = key.replace("_", "-").encode("utf-8")
     headers_dict = dict(request.scope["headers"])
-    return headers_dict[key].decode("utf-8")
+    return headers_dict.get(key, b"").decode("utf-8")
 
 
 def actor(key, request):

tests/test_canned_queries.py

@@ -186,6 +186,7 @@ def magic_parameters_client():
     [
         ("_actor_id", "root"),
         ("_header_host", "localhost"),
+        ("_header_not_a_thing", ""),
         ("_cookie_foo", "bar"),
         ("_now_epoch", r"^\d+$"),
         ("_now_date_utc", r"^\d{4}-\d{2}-\d{2}$"),