SteamWeb and foreign inventory improvements

* Fixed a few crashes caused by foreign inventory craziness
* Improved `SteamWeb` to work with `GET` requests
* Should be able to access "friends-only" inventories now
I'm not sure why this happened, but as I mentioned in another PR, after all the existing PR's are closed, I'll go through and run an auto-formatter over some (all?) of the files.

Otherwise the formatting changes from one PR might conflict with actually-important changes from another.