url_encode: escape url query

For routes such as /api/auth/directory/search, when searching for names such as
"Léo", jams would not respond to the request
since the URL contained invalid characters defined in RFC3986. `url_encode`
turns such characters into their equivalent escaped characters.

include/opendht/http.h

@@ -330,6 +330,8 @@ private:
 
     void build();
 
+    static std::string url_encode(const std::string& value);
+
     void init_default_headers();
     /**
      * Initialized and wraps the http_parser callbacks with our user defined callbacks.

src/http.cpp

@@ -33,6 +33,10 @@
 #include <openssl/x509.h>
 #include <openssl/x509_vfy.h>
 
+#include <cctype>
+#include <iomanip>
+#include <sstream>
+
 #define MAXAGE_SEC (14*24*60*60)
 #define JITTER_SEC (60)
 #define OCSP_MAX_RESPONSE_SIZE (20480)
@@ -1048,7 +1052,7 @@ Request::build()
     bool append_body = !body_.empty();
 
     // first header
-    request << header_.method().c_str() << " " << header_.request_target() << " " <<
+    request << header_.method().c_str() << " " << url_encode(header_.request_target()) << " " <<
                "HTTP/" << header_.http_major() << "." << header_.http_minor() << "\r\n";
 
     // other headers
@@ -1084,6 +1088,30 @@ Request::build()
     request_ = request.str();
 }
 
+// https://stackoverflow.com/a/17708801
+static std::string
+Request::url_encode(const std::string& value)
+{
+    std::ostringstream escaped;
+    escaped.fill('0');
+    escaped << std::hex;
+
+    for (const char& c : value) {
+        // Keep alphanumeric and other accepted characters intact
+        if (std::isalnum(c) || c == '-' || c == '_' || c == '.' || c == '~') {
+            escaped << c;
+            continue;
+        }
+
+        // Any other characters are percent-encoded
+        escaped << std::uppercase;
+        escaped << '%' << std::setw(2) << static_cast<int>(static_cast<unsigned char>(c));
+        escaped << std::nouppercase;
+    }
+
+    return escaped.str();
+}
+
 void
 Request::add_on_status_callback(OnStatusCb cb) {
     cbs_.on_status = std::move(cb);