Including `as_tuple.hpp` fails under clang 3.6

[anarthal]

Version: Boost Asio, Boost develop branch
System: Linux, clang 3.6, C++11, debug

It emits the following error:

In file included from libs/mysql/test/integration/test/snippets/connection_pool.cpp:15:
./boost/asio/as_tuple.hpp:140:28: error: default initialization of an object of const type 'const boost::asio::partial_as_tuple' without a user-provided default constructor
constexpr partial_as_tuple as_tuple;
                           ^

./boost/asio/as_tuple.hpp:140:36: note: add an explicit initializer to initialize 'as_tuple'
constexpr partial_as_tuple as_tuple;
                                   ^
                                   {}

Looks like sticking a brace initializer to the new constexpr variable should fix the issue.

Full build log: https://drone.cpp.al/boostorg/mysql/732/12/4