[#299] client: Do not use pointers to required response fields

In previous implementation `client` package provided access to nested
response fields as pointers to them. This caused clients to handle nil
cases even when the field presence in the response is required.

Avoid returning pointers to required fields in response getters. This
also reduces reference counter load and allows fields to be decoded
directly without additional assignment.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

client/common.go

@@ -251,7 +251,7 @@ func (x *contextCall) close() bool {
 		x.result(x.resp)
 	}
 
-	return true
+	return x.err == nil
 }
 
 // goes through all stages of sending a request and processing a response. Returns true if successful.