If `Content-Type` header is set, verify it's the expected one

[pypt]

For example, if Content-Type for /robots.txt is text/html (and not text/plain), this usually means that the file is missing (and instead a 404 page would get returned) so there's no need to attempt to parse it.

Same goes for XML files, plain text sitemaps, and gzipped XML / text sitemaps.

[tanweeralii]

Hey @pypt, I am a beginner here and I am looking for an good first issue. Seems this issue is good to start with. I have compiled and build it in my system. Now can you just brief more about this issue like how to solve. Thanks