TypeDescription toJson() returns invalid json

toJson method returns invalid JSON string when the category field is STRUCT.

 Inside printJsonToBuffer method, we are careting invalid json array just like below,

{
 "fields":[
    "f1":{},
     "f2":{}
 ]
}

But it should be like this:

{
 "fields":[
    {"f1":{}},
    {"f2":{}}
 ]
}