bug: Incorrect content-type header in Admin API Responses

[shenal]

Issue description

APISIX Admin API sets the content-type as text/plain when the actual content-type is application/json

Environment

• apisix version (cmd: apisix version): master
• OS: ubuntu

Minimal test code / Steps to reproduce the issue

1. Send any valid request to APISIX Admin API
2. Response headers contain content-type as text/plain

What's the actual result? (including assertion message & call stack if applicable)

Response Headers contain Content-Type: text/plain

What's the expected result?

Response Headers should contain Content-Type: application/json