trim trailing slashes from base url

[0robustus1]

When using a custom Github URL (e.g. for a Github Enterprise instance)
one might specify it with a trailing slash. This can lead to URLs
containing two successive slashes.

Github will not ignore this, but instead return a HTTP 401 Forbidden
response. It might be nice to address the issue by removing trailing
slashes when joining with a string that has a leading slash.

[bradleyfalzon]

Thanks for the clean and simple PR. Change makes sense.

I’m on mobile so I didn’t have a clear look at anywhere else we’d likely need to make this change, I presume this was the only location and it’s the best location to make the change?

[0robustus1]

Yes that seemed like the most fitting place considering that it's the only usage in a request and that BaseURL is an exported field on Transport that can be changed at any time.