Make the request_context be able to generate a querystring for a subset.

The request_context can now take a filter to select arguments to
keep in the query string.

src/server/request_context.cpp

@@ -189,18 +189,6 @@ std::string RequestContext::get_header(const std::string& name) const {
   return headers.at(lcAll(name));
 }
 
-std::string RequestContext::get_query() const {
-  std::string q;
-  const char* sep = "";
-  for ( const auto& a : arguments ) {
-    for (const auto& v: a.second) {
-      q += sep + a.first + '=' + v;
-      sep = "&";
-    }
-  }
-  return q;
-}
-
 std::string RequestContext::get_user_language() const
 {
   try {

src/server/request_context.h

@@ -91,7 +91,26 @@ class RequestContext {
     std::string get_url() const;
     std::string get_url_part(int part) const;
     std::string get_full_url() const;
-    std::string get_query() const;
+
+    std::string get_query() const {
+      return get_query([](const std::string& key) {return true;});
+    }
+
+    template<class F>
+    std::string get_query(F filter) const {
+      std::string q;
+      const char* sep = "";
+      for ( const auto& a : arguments ) {
+        if (!filter(a.first)) {
+          continue;
+        }
+        for (const auto& v: a.second) {
+          q += sep + a.first + '=' + v;
+          sep = "&";
+        }
+      }
+      return q;
+    }
 
     ByteRange get_range() const;