Add ContinueWith fetch like 'Map' extensions

[DalSoft]

Similar to Verify, used for formatting response.

var client = new RestClient("https://jsonplaceholder.typicode.com/");

var response = await client.Resource("users/1").Get()
.Then(r => r.EnsureStatusCode)
.Catch(e => e) // Action
.Catch(e => new Error())
.Map() // Casts

Dynamic with func

Last cast is what is returned.