Add has_content_length_impl to Fields (API Change):

* The function has_content_length_impl is added to the
  requirements of Fields.

Actions Required:

* Implement has_content_length_impl for user-defined Fields

test/doc/exemplars.cpp

@@ -224,6 +224,11 @@ protected:
     bool
     get_keep_alive_impl(unsigned version) const;
 
+    /** Returns `true` if the Content-Length field is present.
+    */
+    bool
+    has_content_length_impl() const;
+
     /** Set or clear the method string.
 
         @note Only called for requests.