How can i send html data in email to a customer's ticket using deluge.

Can anyone just help me out from this problem. I want to send an email over a customer's ticket. I want that customer should get the executed html code. But i'm not able to send it. Customer is getting plain text with all html elements. like this:- <div style="font-size: 14px; color: red">Hello</div>. I want customer should only receive Hello with red color fonts.

data_text = '<div style="font-size:14px; color:red">Hello</div>';
param = map();
param.put("ticketStatus", "Closed");
param.put("channel" , "EMAIL");
param.put("to",xyz@abc.com);
param.put("fromEmailAddress", "abc@xyz.com");
// param.put("contentType", "plainText");
param.put("content", data_text);
param.put("isForward", "true");
header_data=map();
header_data.put("Content-Type", "text/html");
result = invokeurl
[
url: "https://desk.zoho.com/api/v1/tickets/"+id+"/sendReply"
type: POST
parameters: param.toString()
headers: header_data
connection:"zohodesk"
];
info result;