smart_auto_mappings: use C-r instead of expr-mapping

This makes it work in tests.  Using `<expr>` instead will not trigger
the expansion of the mapping.
This is likely a bug in vim-vspec; I have tried 1.4.1 (currently pinned)
and 1.6.1.

ftplugin/python/jedi.vim

@@ -38,7 +38,7 @@ if g:jedi#auto_initialization
     endif
 
     if g:jedi#smart_auto_mappings == 1
-        inoremap <buffer> <expr> <Space> jedi#smart_auto_mappings()
+        inoremap <buffer> <space> <C-R>=jedi#smart_auto_mappings()<CR>
     end
 
     if g:jedi#auto_close_doc