This commit works around an issue in std::initializer_list design.
By using a detail::json_ref proxy with a mutable value inside,
rvalue-ness of an input to list initializer is remembered and
used later to move from the proxy instead of copying.

std::initializer_list does not own the temporaries created in
its initialization. Therefore, storing it in an independent
stack variable is unsafe.

C++ overload resolution/list initialization rules are hard.

On MSVC compiler, temporaries that are constructed during a
list initialization, are sometimes destroyed even before calling
the initializing constructor, instead of at the end of the
containing full-expression. This is clearly non-conforming to
[class.temporary].
As the impact of this bug is silently producing incorrect
JSON values, move eagerly from rvalues to be safe.

See https://stackoverflow.com/questions/24586411