Missing key should throw KeyError

It should not throw a NameError, but should throw a KeyError.

Fixes #26278

actionpack/lib/action_dispatch/http/headers.rb

@@ -86,7 +86,7 @@ module ActionDispatch
         @req.fetch_header(env_name(key)) do
           return default unless default == DEFAULT
           return yield if block_given?
-          raise NameError, key
+          raise KeyError, key
         end
       end
 

actionpack/test/dispatch/header_test.rb

@@ -158,4 +158,10 @@ class HeaderTest < ActiveSupport::TestCase
     assert_equal({ "HTTP_REFERER"=>"http://example.com/",
                   "CONTENT_TYPE"=>"text/plain" }, env)
   end
+
+  test "fetch exception" do
+    assert_raises KeyError do
+      @headers.fetch(:some_key_that_doesnt_exist)
+    end
+  end
 end