fix inconsistent choice of init list constructor

On some compilers value{x} invokes initializer_list constructor, on
others it is equivalent to value(x). This is very problematic, but this
isn't something we can fix. On the other hand, we CAN make init list
construction to be equivalent to value(x), if the size of init list is
one. This commit does exactly that.

include/boost/json/impl/value.ipp

@@ -231,13 +231,21 @@ value(
  storage_ptr sp)
 {
  if(value_ref::maybe_object(init))
+ {
  ::new(&obj_) object(
  value_ref::make_object(
  init, std::move(sp)));
- else
+ }
+ else if( init.size() != 1 )
+ {
  ::new(&arr_) array(
  value_ref::make_array(
  init, std::move(sp)));
+ }
+ else
+ {
+ ::new(this) value( init.begin()->make_value( std::move(sp) ) );
+ }
 }
 
 //----------------------------------------------------------

test/value.cpp

@@ -2131,6 +2131,11 @@ class value_test
  void
  testInitList()
  {
+ {
+ value jv{0};
+ BOOST_TEST( jv == 0 );
+ }
+
  check_array(value{0,0,0}, 0, 0, 0);
  check_array(value{false,false,false}, false, false, false);
  check_array(value{false,2,false}, false, 2, false);