I'm astonished to see so many replies requiring coding to change a single use case (GET) in one API instead of using a proper tool what has to be installed once and can be used for any API (own or 3rd party) and all use cases.
So the good answer is:
- If you only want to request json or other content type install Requestly or a similar tool and modify the Accept header.
- If you want to use POST too and have nicely formatted json, xml, etc. use a proper API testing extension like Postman or ARC.