Add missing string parameter converter

SimpleHttpServer/HttpServer.cs

@@ -57,6 +57,7 @@ public sealed class HttpServer {
         void RegisterConverter<T>() where T : IParsable<T> {
             stringToTypeParameterConverters.Add(typeof(T), new ParsableParameterConverter<T>());
         }
+        stringToTypeParameterConverters.Add(typeof(string), new StringParameterConverter());
 
         stringToTypeParameterConverters.Add(typeof(bool), new BoolParsableParameterConverter());
         RegisterConverter<char>();