Commented Issue: Round tripping utf conversion fails for some surrogate pairs [392]

Round tripping utf conversions fails for some surrogate pairs (below test). Starting with 0xD840, 0xDC00 which returns 0xD800, 0xDC00 after the converstions back and forth. Using version 2.6 on Windows. The test passed through on version 2.3

```
#include "stdafx.h"
#include "CppUnitTest.h"
#include <cpprest/json.h>

using namespace Microsoft::VisualStudio::CppUnitTestFramework;

namespace UnitTest
{
TEST_CLASS(UtfConversionTest)
{
void IsConvertedCharacterEqualToOriginal(const std::wstring& original)
{
using namespace utility::conversions;
const auto converted = to_utf16string(to_utf8string(original));
Assert::IsTrue(original == converted);
}
public:

TEST_METHOD(UtfConversionExhaustiveTest)
{
// This test is crude -- it covers all potential Unicode code points,
// without regard to semantics, if they are even defined, or anything.
for (wchar_t c = 1; c < 0xd800u; ++c)
{
IsConvertedCharacterEqualToOriginal(std::wstring(1, c));
}
for (wchar_t c = 0xe000u; c < 0xfffeu; ++c)
{
IsConvertedCharacterEqualToOriginal(std::wstring(1, c));
}
// surrogate pairs
for (wchar_t highSurrogate = 0xd800; highSurrogate <= 0xdbff; ++highSurrogate)
{
for (wchar_t lowSurrogate = 0xdc00; lowSurrogate <= 0xdfff; ++lowSurrogate)
{
std::wstring c;
c.push_back(highSurrogate);
c.push_back(lowSurrogate);
IsConvertedCharacterEqualToOriginal(c);
}
}
}
};
}
```
Comments: A fix has been added in the development branch. Thanks for reporting the issue, will be release 2.7.0. Steve