ResolveRelativePath - Paths that don't start with a slash shouldn't end up with one

Follow up to both #3134 and #3093

Testing on Android constructing a Song from an xnb, we pass in the parameters:
```SimpullsContentAndroid/Music/inGame```
```inGame.mp4```

Currently this ends up as ```/SimpullsContentAndroid/Music/inGame.mp4```, which will be interpreted as coming from the root directory, which is wrong.
I suggest renaming `hasForwardSlash` to `startsWithSlash`.