[5.1] SEF: Don't redirect for documents other than HTML

[Hackwar]

Summary of Changes

With the new options to improve the routing, we introduced redirects when the URL does not meet our expectations. These redirects only take effect when the request is for a frontend page and when it is a GET request. However this also means that for example AJAX calls or calls to JSON/XML output gets redirected. This isn't an issue because GET requests do automatically follow redirects, but it would mean that such a request would be received, redirected and then received again, thus creating 2 requests on the server. In order to reduce load, this PR thus excludes all requests which aren't for HTML documents.

Testing Instructions

1. Set the SEF plugin to remove a trailing slash
2. Go to a category view in the frontend and add /?format=feed to the URL

Actual result BEFORE applying this Pull Request

You get redirected to the URL without the trailing slash and then receive the feed.

Expected result AFTER applying this Pull Request

You get the feed output directly without a redirect.

Link to documentations

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

[Fedik]

I have tested this item ✅ successfully on bc8d43c

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[SniperSister]

I have tested this item ✅ successfully on bc8d43c

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[alikon]

RTC

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[LadySolveig]

Thank you @Hackwar 🚀 and also for testing @Fedik @SniperSister