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

include/boost/beast/http/impl/fields.ipp

@@ -938,6 +938,14 @@ get_keep_alive_impl(unsigned version) const
         it->value()}.exists("close");
 }
 
+template<class Allocator>
+bool
+basic_fields<Allocator>::
+has_content_length_impl() const
+{
+    return count(field::content_length) > 0;
+}
+
 template<class Allocator>
 inline
 void