Handle unexpected behavior with URI#merge and subpaths missing trailing slashes

[drcapulet]

Due to how URI#merge works (which URI#+ is aliased to), when using a source under a subpath, it can get overwritten:

> URI.parse('https://gems.example.com/private') + './api/v1/dependencies'
=> #<URI::HTTPS https://gems.example.com/api/v1/dependencies>
> URI.parse('https://gems.example.com/private/') + './api/v1/dependencies'
=> #<URI::HTTPS https://gems.example.com/private/api/v1/dependencies>

It's not immediately obvious that errors fetching would be caused by a missing slash, especially if you have a server mounted at both / and /private.

This PR makes sure that calls to URI#+ are more obvious and subpaths aren't overwritten and instead appended.

Fixes #1829.

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[bronzdoc]

Hey @drcapulet! could you add the test into test/rubygems/test_gem_source.rb? thank you!

[drcapulet]

@bronzdoc Was there something you'd want me to cover in test/rubygems/test_gem_source.rb that isn't in test/rubygems/test_gem_source_subpath_problem.rb? Or do you mean try merging them together? If so, I wasn't sure on a good way to achieve what I wanted with #setup.

[bronzdoc]

@drcapulet sorry for the confusion. I meant "move" not "add"

[hsbt]

@drcapulet Hi, Can you move the your test into the test_gem_source.rb?

After that, I will merge this.

[drcapulet]

@hsbt As I mentioned earlier, I wasn't sure of a good way to set @gem_repo - do you just want me to copy the setup across the three tests?

[deivid-rodriguez]

@hsbt Should @drcapulet copy the test setup across each of the tests or do you have another preferred way? In my opinion, a separate test file like it is coded currently is fine.

By the way, does this fix or is related to #1829?

[drcapulet]

@deivid-rodriguez Yes, this should fix #1829.

[deivid-rodriguez]

Great! Could you rebase this PR to make sure everything is still good, and so that the PR is ready to merge in case @hsbt is fine with it now?

[deivid-rodriguez]

I rebased this PR and added my suggestions. I'll merge in a couple of days unless there's further feedback.

[deivid-rodriguez]

Thanks @drcapulet!