"Couldn't connect to Etherpad" error

### Observed effect:

After typing `:Etherpad https://riseup.net/p/foo` in neovim, I get this error message:

`Couldn't connect to Etherpad: https://pad.riseup.net:443/p/foo`

### Expected effect:

It should open up an Etherpad at that address.

### How to reproduce:

1. Install neovim
2. Install the plugin manager https://github.com/junegunn/vim-plug
3. Install any vim-etherpad python dependencies with pip2
4. Install vim-etherpad by inserting `Plug 'guyzmo/vim-etherpad'` into ~/.config/nvim/init.vim and then running :PlugInstall in nvim
5. If step 4 produces an error about a missing module, use pip2 to install any missing modules and repeat step 4 and 5 until there are no more errors.
6. Type `:Etherpad https://pad.riseup.net/p/foo` in neovim.

### Versions, OS, and other useful information

```
$ nvim --version
NVIM v0.2.2
Build type: RelWithDebInfo
LuaJIT 2.1.0-beta3
Compilation: /usr/bin/cc -g -O2 -fdebug-prefix-map=/build/neovim-_mzR4z/neovim-0.2.2=. -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -DDISABLE_LOG -Wdate-time -D_FORTIFY_SOURCE=2 -Wconversion -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=1 -DNVIM_MSGPACK_HAS_FLOAT32 -DNVIM_UNIBI_HAS_VAR_FROM -O2 -g -DMIN_LOG_LEVEL=3 -Og -g -Wall -Wextra -pedantic -Wno-unused-parameter -Wstrict-prototypes -std=gnu99 -Wimplicit-fallthrough -Wvla -fstack-protector-strong -fdiagnostics-color=auto -DINCLUDE_GENERATED_DECLARATIONS -D_GNU_SOURCE -I/build/neovim-_mzR4z/neovim-0.2.2/build/config -I/build/neovim-_mzR4z/neovim-0.2.2/src -I/usr/include -I/usr/include -I/usr/include -I/usr/include -I/usr/include -I/usr/include -I/usr/include -I/build/neovim-_mzR4z/neovim-0.2.2/build/src/nvim/auto -I/build/neovim-_mzR4z/neovim-0.2.2/build/include
Compiled by pkg-vim-maintainers@lists.alioth.debian.org

Features: +acl +iconv +jemalloc +tui
See ":help feature-compile"

system vimrc file: "$VIM/sysinit.vim"
fall-back for $VIM: "/usr/share/nvim"

Run :checkhealth for more info

$ cat ~/.config/nvim/init.vim
call plug#begin('~/.local/share/nvim/plugged')

Plug 'jamessan/vim-gnupg'
Plug 'guyzmo/vim-etherpad'

call plug#end()
```

Latest version of vim-plug is installed, i'm trying to paste a link to the latest commit or a commit hash but it's not working some reason.