begin refactor of proxy engine

proxychain/rqm/modify_query_params.go

@@ -0,0 +1,20 @@
+package rqm // ReQuestModifier
+
+import (
+	"ladder/proxychain"
+)
+
+// ModifyQueryParams replaces query parameter values in URL's query params in a ProxyChain's URL.
+// If the query param key doesn't exist, it is created.
+func ModifyQueryParams(key string, value string) proxychain.RequestModification {
+	return func(px *proxychain.ProxyChain) error {
+		q := px.Request.URL.Query()
+		if value == "" {
+			q.Del(key)
+			return nil
+		}
+		q.Set(key, value)
+		px.Request.URL.RawQuery = q.Encode()
+		return nil
+	}
+}