Fix uninitialized member struct

This caused a program crash in std::string() ctor when got no response
from the remote server (the text_length field of response_ was not
zero-initialized).
Don't print response buffer contents on error
Update year
Fix Boost warning regarding target Windows version

Fix the following warning:

Please define _WIN32_WINNT or _WIN32_WINDOWS appropriately
Assuming _WIN32_WINNT=0x0501 (i.e. Windows XP target)

by defining _WIN32_WINNT=0x0501 as suggested.