Revert "Use shiftwidth 0 instead of copying tabstop"

This reverts commit b6b4c3b237678f1214ea2eca0e3e50eaafb30e3a.

Closes #25.

README.markdown

@@ -28,8 +28,7 @@ then simply copy and paste:
   the first file in a brand new Ruby project might very well be derived from
   your `.irbrc`.  I consider this a feature.
 * If your file is consistently indented with hard tabs, `'shiftwidth'` will be
-  set to 0 so that `'tabstop'` is used.  Otherwise, a `'tabstop'` of 8 is
-  enforced.
+  set to your `'tabstop'`.  Otherwise, a `'tabstop'` of 8 is enforced.
 * The algorithm is rolled from scratch, fairly simplistic, and only lightly
   battle tested.  It's probably not (yet) as good as [DetectIndent][].
   Let me know what it fails on for you.

plugin/sleuth.vim

@@ -67,11 +67,7 @@ function! s:guess(lines) abort
   endfor
 
   if heuristics.hard && !heuristics.spaces
-    if exists('*shiftwidth')
-      return {'expandtab': 0, 'shiftwidth': 0}
-    else
-      return {'expandtab': 0, 'shiftwidth': &tabstop}
-    endif
+    return {'expandtab': 0, 'shiftwidth': &tabstop}
   elseif heuristics.soft != heuristics.hard
     let options.expandtab = heuristics.soft > heuristics.hard
     if heuristics.hard