CSHARP-3037: Correcting types in ArgumentException message.

[kevbite]

Fixing expectation message whereby the wrong type is contained within the exception making it confusing to the end-user.

Take for example the following test:

[Fact]
public void Test1()
{
    PipelineDefinition<BsonDocument, MyType>.Create(new IPipelineStageDefinition[]
    {
        new JsonPipelineStageDefinition<BsonDocument, BsonDocument>(
            @"{ project: { } } "),
    });
}

public class MyType { }

This throws the following exception

System.ArgumentException : The output type to the last stage was expected to be MongoDB.Bson.BsonDocument, but was MongoDB.Bson.BsonDocument. (Parameter 'stages')

instead of

System.ArgumentException : The output type to the last stage was expected to be SomeExample.MyType, but was MongoDB.Bson.BsonDocument. (Parameter 'stages')

[vincentkam]

Created https://jira.mongodb.org/browse/CSHARP-3037 to track this 😺

[vincentkam]

Thank for your PR @kevbite! I've merged your change with some small tweaks here: cddc136 😸