fix: Set X-Requested-With header on all requests to avoid browser auth dialogs

[juliushaertl]

Server has logic in place to return a dummyauth response header in case this one is set: https://github.com/nextcloud/server/commit/dfc3536d2b95fea1b54b3e85651e3a66c2d0088e

This helps to handle requests that happen in the web UI after authentication was revoked (e.g. by disabling a user) more gracefully, otherwise a request would lead to the browser showing a basic auth dialog.

Steps to reproduce:

• Login as a user
• Disable the user through occ
• Wait until the notifications app sends another request

[codecov[bot]]

Codecov Report

Merging #637 (ceb4dc3) into master (27e2158) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #637   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            4         4           
  Lines           40        40           
  Branches         5         5           
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  Hits            40        40           

Files Changed	Coverage Δ
lib/index.ts	100.00% <ø> (ø)