/source redirects to /source.pdf for PDF source files

[longhotsummer]

This ensures that documents that are pure PDF are only served from a single URL (/source.pdf) which is good for caching, google and pocketlaw.

Also harmonises download headers for /source and /source.pdf

Non-PDF:

$ http -p Hh http://localhost:8000/akn/ug/judgment/ughc/2009/165/eng@2009-09-29/source
GET /akn/ug/judgment/ughc/2009/165/eng@2009-09-29/source HTTP/1.1
Accept: */*
Accept-Encoding: gzip, deflate
Connection: keep-alive
Host: localhost:8000
User-Agent: HTTPie/3.2.1

HTTP/1.1 200 OK
Cache-Control: max-age=86400
Content-Disposition: inline; filename=In Re Resty Nanyanzi and Joyce Kisakye (Infants) (Family Cause No 122 of 2009) 2009 UGHC 165 (29 September 2009).rtf
Content-Language: en
Content-Length: 53663
Content-Type: application/rtf
...

---

$ http -p Hh http://localhost:8000/akn/ug/judgment/ughc/2009/165/eng@2009-09-29/source.pdf
GET /akn/ug/judgment/ughc/2009/165/eng@2009-09-29/source.pdf HTTP/1.1
Accept: */*
Accept-Encoding: gzip, deflate
Connection: keep-alive
Host: localhost:8000
User-Agent: HTTPie/3.2.1

HTTP/1.1 200 OK
Cache-Control: max-age=86400
Content-Disposition: inline; filename=In Re Resty Nanyanzi and Joyce Kisakye (Infants) (Family Cause No 122 of 2009) 2009 UGHC 165 (29 September 2009).pdf
Content-Language: en
Content-Length: 71890
Content-Type: application/pdf
...

PDF:

$ http -p Hh http://localhost:8000/akn/za/judgment/foo/2023/1/eng@2023-01-01/source
GET /akn/za/judgment/foo/2023/1/eng@2023-01-01/source HTTP/1.1
Accept: */*
Accept-Encoding: gzip, deflate
Connection: keep-alive
Host: localhost:8000
User-Agent: HTTPie/3.2.1

HTTP/1.1 302 Found
Content-Language: en
Content-Length: 0
Content-Type: text/html; charset=utf-8
Date: Wed, 26 Jul 2023 09:21:41 GMT
Location: /akn/za/judgment/foo/2023/1/eng@2023-01-01/source.pdf
...

---

$ http -p Hh http://localhost:8000/akn/za/judgment/foo/2023/1/eng@2023-01-01/source.pdf
GET /akn/za/judgment/foo/2023/1/eng@2023-01-01/source.pdf HTTP/1.1
Accept: */*
Accept-Encoding: gzip, deflate
Connection: keep-alive
Host: localhost:8000
User-Agent: HTTPie/3.2.1

HTTP/1.1 200 OK
Cache-Control: max-age=86400
Content-Disposition: inline; filename=test for pdf 2023 FOO 1 (1 January 2023).pdf
Content-Language: en
Content-Length: 255768
Content-Type: application/pdf
...