Preserve path part of baseUri when building new URIs

[zhongfu]

currently, the path part of the provided server URI (or base URI) is overwritten when new URIs are built, which leads to unexpected behavior, e.g.

• provided server URI: https://blah.example/qbt/ or https://blah.example/qbt
• expected URI for v2 api login endpoint: https://blah.example/qbt/api/v2/auth/login
• actual URI: https://blah.example/api/v2/auth/login

this PR makes it such that the path part of the server URI is preserved when building new URIs (or at least it did when I tested this change)

should fix fedarovich/qbittorrent-cli#30 fedarovich/qbittorrent-cli#49

[fedarovich]

I have fixed the corresponding issue in a bit different way.