fix(filters): KVFilter process one field source

KVFilter do not process one field source like 'foo=bar', now it split it
into {"foo":"bar"}

filter/kv.go

@@ -91,9 +91,6 @@ func (p *KVFilter) Filter(event map[string]interface{}) (map[string]interface{},
 		return event, false
 	}
 	A := strings.Split(msg.(string), p.field_split)
-	if len(A) == 1 {
-		return event, false
-	}
 
 	var o map[string]interface{} = event
 	if p.target != "" {

filter/kv_test.go

@@ -103,3 +103,24 @@ func TestKVFilter(t *testing.T) {
 		t.Error("kv failed")
 	}
 }
+func TestKVFilterOneField(t *testing.T) {
+	config := make(map[interface{}]interface{})
+	config["field_split"] = " "
+	config["value_split"] = "="
+	config["src"] = "message"
+	f := BuildFilter("KV", config)
+
+	event := make(map[string]interface{})
+	event["message"] = "a=aaa"
+	t.Log(event)
+
+	event, ok := f.Filter(event)
+	if !ok {
+		t.Error("kv failed")
+	}
+	t.Log(event)
+
+	if event["a"] != "aaa" {
+		t.Error("kv failed")
+	}
+}