fix missing header detection for the non-strict case

responses/matchers.py

@@ -421,7 +421,7 @@ def _compare_with_regex(request_headers: Union[Mapping[Any, Any], Any]) -> bool:
                 else:
                     if not v == request_headers[k]:
                         return False
-            elif strict_match:
+            else:
                 return False
 
         return True

responses/tests/test_matchers.py

@@ -626,7 +626,7 @@ def run():
     assert_reset()
 
 
-def test_request_matches_headers_no_match():
+def test_request_header_value_mismatch_raises():
     @responses.activate
     def run():
         url = "http://example.com/"
@@ -650,6 +650,27 @@ def run():
     assert_reset()
 
 
+def test_request_headers_missing_raises():
+    @responses.activate
+    def run():
+        url = "http://example.com/"
+        responses.add(
+            method=responses.GET,
+            url=url,
+            json={"success": True},
+            match=[matchers.header_matcher({"x-custom-header": "foo"})],
+        )
+
+        with pytest.raises(ConnectionError) as excinfo:
+            requests.get(url, headers={})
+
+        msg = str(excinfo.value)
+        assert ("Headers do not match: {} doesn't match {x-custom-header: foo}") in msg
+
+    run()
+    assert_reset()
+
+
 def test_request_matches_headers_strict_match():
     @responses.activate
     def run():