Add Content-Type to Shibboleth handler response

[maxbes]

Currently, the Shibboleth handler (/login) does not set a Content-Type:

This can cause issues with certain reverse proxies, certain default Content-Type configurations, or when using X-Content-Type-Options: nosniff . In those cases the HTML source code can appear in the browser window instead of being rendered

This PR adds the missing Content-Type header

[jitsi-jenkins]

Hi, thanks for your contribution!
If you haven't already done so, could you please make sure you sign our CLA (https://jitsi.org/icla for individuals and https://jitsi.org/ccla for corporations)? We would unfortunately be unable to merge your patch unless we have that piece :(.

[maxbes]

Hi, thanks for your contribution!
If you haven't already done so, could you please make sure you sign our CLA (https://jitsi.org/icla for individuals and https://jitsi.org/ccla for corporations)? We would unfortunately be unable to merge your patch unless we have that piece :(.

done