Created Unassigned: Build fails on OS X 10.8.5 due to -Wnull-conversion [327]

Hi,

There is a problem while compiling CPPRest 2.3 on Mac OS X 10.8.5 with a conversion from NULL to nullptr_t:

```
...
cpprest/astreambuf.h:524:42: error: implicit conversion of NULL constant to 'nullptr_t' [-Werror,-Wnull-conversion]
...
```

I patched the CMakeList.txt to add -Wno-conversion-null flag to OSX_SUPPRESSIONS.

Best Regards,