SimpleClient: Make it generic over stream type
SimpleClient: Make it generic over stream type

The `SimpleClient` can now accept any `HttpConnection<S>` and isn't
hardcoded to instantiate a `TcpStream` itself.

stepancheg/solicit,mlalic/solicit,Galvanization/solicit,jinnjuice/solicit,gwicke/solicit,tempbottle/solicit