HttpHelper wrongly passes url as std::string

[Epse]

In HttpHelper, URLs are passed to libcurl as std::string, where they should be char pointers, thus breaking things like CPDLC, due to libcurl errors.
The std::string.c_str() method is exactly what's needed here.

See:

vSMR/vSMR/HttpHelper.cpp

Line 32 in e696e09

curl_easy_setopt(curl, CURLOPT_URL, url);

If some more time can be given, I can prepare a patch to fix this.