Actions Required:
* Change calls to msg.prepare to msg.prepare_payload. For messages
with a user-defined Fields, provide the function prepare_payload_impl
in the fields type according to the Fields requirements.
Every interface or implementation which operates on a templated
type Fields is evaluated to determine if basic_fields<Allocator>
is more appropriate, and changed if so.
* `parser` is now templated on Allocator
* `parser` only produces messages using `basic_fields<Allocator>`
as the Fields type.
* message-oriented read and async_read only work on messages
using basic_fields as the Fields type.
Actions Required:
* Callers using `parser` with Fields types other than basic_fields
will need to create their own subclass of basic_parser to work
with their custom fields type.
fix#297, fix#488
* Errors are checked and reported
* More comments explaining what is going on
* The connection is gracefully closed
WebSocket:
* Messages are drained before closing
fix#497
This changes the function signature to accept the `value_type`
instead of the entire message.
Actions Required:
* For any user-defined models of Body, change the function signature
to accept `value_type const&` and modify the function definition
accordingly.
