Fix compilation error on Linux systems

L"NULL" is a wchar in which fails to compile if the cpprestsdk::string_t is defined as std::string and not std::wstring (It's std::wstring only for windows)

Author: TomerJLevy

samples/client/petstore/cpp-restsdk/client/ModelBase.cpp

@@ -296,7 +296,7 @@ utility::string_t ModelBase::stringFromJson(const web::json::value& val)
 
 utility::datetime ModelBase::dateFromJson(const web::json::value& val)
 {
-    return val.is_null() ? utility::datetime::from_string(L"NULL", utility::datetime::ISO_8601) : utility::datetime::from_string(val.as_string(), utility::datetime::ISO_8601);
+    return val.is_null() ? utility::datetime::from_string(utility::conversions::to_string_t("NULL"), utility::datetime::ISO_8601) : utility::datetime::from_string(val.as_string(), utility::datetime::ISO_8601);
 }
 bool ModelBase::boolFromJson(const web::json::value& val)
 {