Fix session metrics for stream upstreams

* 4xx and 5xx session metrics are not correctly unmarshalled. Instead
they are ignored.

Bug introduced in: c9fb4ac

Author: Dean-Coakley

client/nginx.go

@@ -184,8 +184,8 @@ type Responses struct {
 // Sessions represents stream session related stats.
 type Sessions struct {
 	Sessions2xx uint64 `json:"2xx"`
-	Sessions4xx uint64 `josn:"4xx"`
-	Sessions5xx uint64 `josn:"5xx"`
+	Sessions4xx uint64 `json:"4xx"`
+	Sessions5xx uint64 `json:"5xx"`
 	Total       uint64
 }