* detail::read_size_helper is removed
* public function read_size helps determine the optimum
size for calls to a DynamicBuffer's prepare function.
* special case for Asio types which don't conform to
the concept but have their own helper function.
read_size_helper can return zero if the buffer reaches
its maximum size, causing infinite loops in HTTP. The
function maybe_read_size_helper is provided to throw
an exception instead of returning a value for this case.
