SdfGenerator_TEST failures on master

[scpeters]

There have been several recurring UNIT_SdfGenerator_TEST failures on the master branch since the following build on August 21:

• https://build.osrfoundation.org/job/ignition_gazebo-ci-master-bionic-amd64/24/

The following test failure provides a clue of the difference:

• https://build.osrfoundation.org/job/ignition_gazebo-ci-master-bionic-amd64/24/testReport/junit/(root)/ElementUpdateFixture/WorldWithModelsIncludedExpanded/

The original world file has the following uri:

<uri>https://fuel.ignitionrobotics.org/1.0/openrobotics/models/Backpack/1/files/meshes/Backpack.dae</uri>`

The generated world file appears to have 2 URI's concatenated together:

<uri>file:///root/.ignition/fuel/fuel.ignitionrobotics.org/openrobotics/models/Backpack/1/https://fuel.ignitionrobotics.org/1.0/openrobotics/models/Backpack/1/files/meshes/Backpack.dae</uri>

I added some debugging print statements to the test in scpeters@a1490fb that confirm the other failures are connected to this same issue.

I think this may be related to gazebosim/gz-fuel-tools#85

[scpeters]

@ahcorde noticed this same thing in #314 (comment)

[chapulina]

They should be fixed when #271 is merged forward, see #321

[chapulina]

Latest master branch build is blue: https://build.osrfoundation.org/job/ignition_gazebo-ci-master-bionic-amd64/29/