fix decode data loss when using int64 in json

The problem is when I create container though REST api, set memory limit
in hostConfig, the memory limit didn't work. Because when we DecodeEnv,
we got hostConfig part of Env like this:
{"Binds":["/:/tmp"],"CpuShares":512,"CpusetCpus":"0,1","Devices":[],"Memory":1.6777216e+07,"MemorySwap":0}

And we cannot unmarshal number 1.6777216e+07 into Go value of type int64,
so we got 0.

We can fix this by setting Decoder as UseNumber().

Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>

engine/env.go

@@ -189,7 +189,10 @@ func (decoder *Decoder) Decode() (*Env, error) {
 // is returned.
 func (env *Env) Decode(src io.Reader) error {
 	m := make(map[string]interface{})
-	if err := json.NewDecoder(src).Decode(&m); err != nil {
+	d := json.NewDecoder(src)
+	// We need this or we'll lose data when we decode int64 in json
+	d.UseNumber()
+	if err := d.Decode(&m); err != nil {
 		return err
 	}
 	for k, v := range m {

engine/env_test.go

@@ -78,6 +78,26 @@ func TestSetenv(t *testing.T) {
 	}
 }
 
+func TestDecodeEnv(t *testing.T) {
+	job := mkJob(t, "dummy")
+	type tmp struct {
+		Id1 int64
+		Id2 int64
+	}
+	body := []byte("{\"tags\":{\"Id1\":123, \"Id2\":1234567}}")
+	if err := job.DecodeEnv(bytes.NewBuffer(body)); err != nil {
+		t.Fatalf("DecodeEnv failed: %v", err)
+	}
+	mytag := tmp{}
+	if val := job.GetenvJson("tags", &mytag); val != nil {
+		t.Fatalf("GetenvJson returns incorrect value: %s", val)
+	}
+
+	if mytag.Id1 != 123 || mytag.Id2 != 1234567 {
+		t.Fatal("Get wrong values set by job.DecodeEnv")
+	}
+}
+
 func TestSetenvBool(t *testing.T) {
 	job := mkJob(t, "dummy")
 	job.SetenvBool("foo", true)