Implemented Dispose(bool)

Added .nvmrc

Author: alexeyzimarev

src/RestSharp/.nvmrc

@@ -0,0 +1 @@
+v17.0.1

src/RestSharp/RestClient.cs

@@ -193,10 +193,18 @@ void DoBuildUriValidations(RestRequest request) {
             );
     }
 
+    readonly bool _disposeHttpClient;
+    bool          _disposed;
+
+    protected virtual void Dispose(bool disposing) {
+        if (disposing && !_disposed) {
+            _disposed = true;
+            if (_disposeHttpClient) HttpClient.Dispose();
+        }
+    }
+
     public void Dispose() {
-        if (_disposeHttpClient) HttpClient.Dispose();
+        Dispose(true);
         GC.SuppressFinalize(this);
     }
-
-    readonly bool _disposeHttpClient;
 }