Fix Pathname compatibility.

[voxik]

Until PR #42 was merged, String, as well as Pathname objects, were supported just fine. Now, just String is supported, because Pathname object does not support #start_with? method.

Therefore, this just naively converts everything to String to fix Pathname support. I can imagine, that it would use Pathname#absolute? if available, but not sure what would be the result.

Please note that this fixes "4) Vagrant::Action::Builtin::BoxAdd with box file directly adds from FTP URL Failure/Error: server.add_file(path.basename, path.read)" failure mentioned in hashicorp/vagrant/issues/10646

[voxik]

Just FTR, this is full backtrace from the Vagrant test case:

  1) Vagrant::Action::Builtin::BoxAdd with box file directly adds from FTP URL
     Failure/Error: server.add_file(path.basename, path.read)
     
     NoMethodError:
       undefined method `start_with?' for #<Pathname:temporary.box>
     # /builddir/.gem/ruby/gems/fake_ftp-0.3.0/lib/fake_ftp/server.rb:157:in `abspath'
     # /builddir/.gem/ruby/gems/fake_ftp-0.3.0/lib/fake_ftp/server.rb:62:in `add_file'
     # ./test/unit/vagrant/action/builtin/box_add_test.rb:52:in `with_ftp_server'
     # ./test/unit/vagrant/action/builtin/box_add_test.rb:151:in `block (3 levels) in <top (required)>'