fix url_encode for packages with underscores

fixed the url_encode function for packages with underscores.

    $ aurget --debug acpi_call-git
    [DEBUG] setting up 1 targets for processing
    :: Searching AUR...
    [DEBUG] HTTP GET https://aur.archlinux.org/rpc.php?type=multiinfo&arg\[\]=acpi0.000000call-git
    error: target not found: acpi_call-git