patch 8.0.1455: if $SHELL contains a space then 'shell' is incorrect

Problem:    If $SHELL contains a space then the default value of 'shell' is
            incorrect. (Matthew Horan)
Solution:   Escape spaces in $SHELL. (Christian Brabandt, closes #459)
diff --git a/src/testdir/test_startup.vim b/src/testdir/test_startup.vim
index 6f67bfc..42e4896 100644
--- a/src/testdir/test_startup.vim
+++ b/src/testdir/test_startup.vim
@@ -226,6 +226,20 @@
   call delete('Xtestout')
 endfunc
 
+func Test_set_shell()
+  let after = [
+	\ 'call writefile([&shell], "Xtestout")',
+	\ 'quit!',
+	\ ]
+  let $SHELL = '/bin/with space/sh'
+  if RunVimPiped([], after, '', '')
+    let lines = readfile('Xtestout')
+    " MS-Windows adds a space after the word
+    call assert_equal('/bin/with\ space/sh', lines[0])
+  endif
+  call delete('Xtestout')
+endfunc
+
 func Test_progpath()
   " Tests normally run with "./vim" or "../vim", these must have been expanded
   " to a full path.