Multiple env vars in a line causes executing error

```http
POST https://httpbin.org/post
Content-Type: application/x-www-form-urlencoded

a={{VAR_1}}&b={{VAR_2}}
```

message:

```
E5108: Error executing lua .../Nil/.vim/plugged/rest.nvim/lua/rest-nvim/utils/init.lua:74: bad argument #2 to 'gsub' (string/function/table expected)
```