No const operator[]

If you try to examine a const JSONVar object using operator[] (e.g. auto foo = myVar["bar"]), the compiler will (obviously) not be able locate the nonexistent const operator[], but it won't quit there. Instead, it will (silently) beat your JSONVar sword into some unrelated plowshare and apply type type's const operator[] , with predictably nonsensical results.
For example, this code:

const JSONVar myVar1 = JSON.parse( "{\"foo\": \"bar\"}" );
auto myVar2 = myVar1[ "foo" ];
Serial.print( "foo: " );
Serial.println( myVar2 );

produces this output:
foo: f

If I add this method to JSONVar:

JSONVar JSONVar::operator[]( const char* key ) const
{
	if ( !cJSON_IsObject( _json ) )
	{
		return undefined;
	}
	cJSON* json = cJSON_GetObjectItemCaseSensitive( _json, key );
	if ( json == NULL )
	{
		json = cJSON_AddNullToObject( _json, key );
	}
	return JSONVar( json, _json );
}

then the output changes to a more reasonable
foo: "bar"