DefaultParameters collection is not read-only

[alexeyzimarev]

The DefaultParameters property of RestClient has the ParametersCollection type. Although it's a read-only collection, it exposes public methods to add and remove parameters.

When using RestClient.AddDefaultParameter method, the collection is updated in a locked scope. However, it is possible to manipulate the collection directly by calling client.DefaultParameters.AddParameter, and it will be execute without locking the collection. It basically makes the client not thread-safe if default parameters are updated this way. It's not an intended use of the collection, but it has a public API, which allows people to do that.

My suggestion is to make the DefaultParameters property internal or private. The AddDefaultParameter might need to be added to the interface.