Rework the body of request and response

webcc/common.h

@@ -79,6 +79,8 @@ public:
 
   void Parse(const std::string& str);
 
+  void Reset();
+
   bool Valid() const;
 
   bool multipart() const {
@@ -209,6 +211,10 @@ public:
   // API: CLIENT
   void Prepare(Payload* payload);
 
+  // Get the payload size.
+  // Used by the request to calculate content length.
+  std::size_t GetSize();
+
 private:
   // Generate headers from properties.
   void SetHeaders();