built-in r-value return values can't be assigned:

This fixes the following warning:

  warning: ‘const’ type qualifier on return type has no effect [-Wignored-qualifiers]

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

@@ -291,7 +291,7 @@ value_type(field name,
 
 template<class Allocator>
 inline
-field const
+field
 basic_fields<Allocator>::
 value_type::
 name() const