use string::length

[paulpach]

in several places you are using strlen(x.c_str())
This is quite inefficient because strlen has to walk the string which is O(n). Instead, std::string has a length method which is O(1)

for example, the following code:

    int n = strlen(http_str.c_str());

would be more efficient and readable like this:

    auto n = http_str.length();