Fix OrRule logic on parameter lookup failures

Fixes #471

internal/hook/hook.go

@@ -848,7 +848,9 @@ func (r OrRule) Evaluate(req *Request) (bool, error) {
 	for _, v := range r {
 		rv, err := v.Evaluate(req)
 		if err != nil {
-			return false, err
+			if !IsParameterNodeError(err) {
+				return false, err
+			}
 		}
 
 		res = res || rv

internal/hook/hook_test.go

@@ -657,13 +657,13 @@ var orRuleTests = []struct {
 	},
 	// failures
 	{
-		"invalid rule",
+		"missing parameter node",
 		OrRule{
 			{Match: &MatchRule{"value", "", "", "z", Argument{"header", "a", "", false}, ""}},
 		},
 		map[string]interface{}{"Y": "Z"}, nil, nil,
 		[]byte{},
-		false, true,
+		false, false,
 	},
 }