provider/lang: expand() g:foo_host_prog (#9312)

Before this commit, if user does this:
    let g:node_host_prog = '~/.nvm/versions/node/v11.3.0/bin/neovim-node-host'
the "~/" is not expanded to user's home directory.

`:help g:ruby_host_prog` suggests a path with "~/" so technically we
already claimed to support this.

closes https://github.com/neovim/node-client/issues/102

runtime/autoload/provider/ruby.vim

@@ -45,7 +45,7 @@ endfunction
 
 function! s:detect()
   if exists("g:ruby_host_prog")
-    return g:ruby_host_prog
+    return expand(g:ruby_host_prog)
   elseif has('win32')
     return exepath('neovim-ruby-host.bat')
   else