Add test case for unescaped spaces in &rtp

Author: junegunn

Diff for: test/regressions.vader

@@ -218,3 +218,11 @@ Execute (#159: shell=/bin/tcsh):
     let &shell = org
   endtry
 
+**********************************************************************
+Execute (#154: Spaces in &rtp should not be escaped):
+  call plug#begin('$TMPDIR/plug it')
+  Plug 'seoul256 vim'
+  call plug#end()
+  Log &rtp
+  Assert stridx(&rtp, 'plug it/seoul256 vim') >= 0
+