url_relative tests fail againts libxml2 2.9.9

[QuLogic]

Issue Description and Expected Result

I'm looking at our automatic rebuilds of R-xml2, and it has started failing recently. The latest build's most significant change is libxml2 2.9.8 -> 2.9.9. Unit tests now fail as:

Running the tests in 'tests/testthat.R' failed.
Last 13 lines of output:
  > 
  > test_check("xml2")
  This is libxml2 version 2.9.9 
  -- 1. Failure: url_relative (@test-url.R#18)  ----------------------------------
  url_relative("http://hadley.nz/a/c", "http://hadley.nz/") not equal to "../a/c".
  1/1 mismatches
  x[1]: "a/c"
  y[1]: "../a/c"
  
  == testthat results  ===========================================================
  OK: 381 SKIPPED: 2 FAILED: 1
  1. Failure: url_relative (@test-url.R#18) 
  
  Error: testthat unit tests failed

I believe GNOME/libxml2@b1f87c0 might be related.

Session Info

It's an automated build, so I can't really get this, sorry. You can check the root.log in the build link above for installed packages.

[QuLogic]

Also causing FTBFS during the Fedora 30 mass rebuild just before the branching point.

[QuLogic]

I've used this patch for now, but obviously, it's not portable.