Fix packing issue for nested types

Currently, the keys for nested maps must be in type interface{}

<pre>
// this will get error:
// interface conversion: interface is map[string]interface {}, not map[interface {}]interface {}
rowList := []map[string]interface{}{}
row := map[string]interface{}{"key": "value"}
rowList = append(rowList, row)
bin := aerospike.NewBin("bin", rowList)

// this will work
rowList := []map[interface{}]interface{}{}
row := map[interface{}]interface{}{"key": "value"}
rowList = append(rowList, row)
bin := aerospike.NewBin("bin", rowList)

</pre>