Fixed XSS security bug

[r0hanSH]

Fix: set Content-Type response header as text/plain rather than default text/html

The actual vulnerability was reported by me on huntr.dev platform.
People having write permissions on this repo can see the actual vulnerability report at:
https://huntr.dev/bounties/986fb54a-9353-43c1-9433-9a54b3d31e6e/

JamieSlome asked you to create SECURITY.md, a month ago. (#1891) for this vulnerability.

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[r0hanSH]

@plouc
can we you please check this security bug?

[Izurii]

@plouc Could take a look at this? I think this PR should be fairly easy to review and an important issue to be looking at, having a security flaw isn't good for anyone

[plouc]

@Izurii, yes it was easy to review, I had a look at it, but the CI is failing, which is also not good for anyone ;).

[plouc]

@r0hanSH, thank you for this fix, could you please also fix the failing tests? (it's a formatting issue).

[Izurii]

@Izurii, yes it was easy to review, I had a look at it, but the CI is failing, which is also not good for anyone ;).

Ohh I didn't see that before. I agree with you, CI failing is not good too. Thanks for the reply

I really liked using this library and I want to see it growing. I used before recharts, react-chartjs-2 and highcharts (too costly), between all that, I'm starting to use in most of my projects this library here.

[r0hanSH]

@plouc I pushed the required changes

[plouc]

Thank you @r0hanSH!

[r0hanSH]

@plouc can you please validate and close this report? https://huntr.dev/bounties/986fb54a-9353-43c1-9433-9a54b3d31e6e/