Substituting "%7E" for "~" fixes a linkcheck error on my machine

Specifically, the following behavior seems a little odd:

```
julia> using Requests

julia> get("http://math.mit.edu/%7Estevenj/Julia-cheatsheet.pdf")
Response(403 Forbidden, 10 Headers, 309 Bytes in Body)

julia> get("http://math.mit.edu/~stevenj/Julia-cheatsheet.pdf")
Response(200 OK, 12 Headers, 85999 Bytes in Body)
```