patch 8.0.0847: :argadd without argument can't handle space in file name

Problem:    :argadd without argument can't handle space in file name. (Harm te
            Hennepe)
Solution:   Escape the space. (Yasuhiro Matsumoto, closes #1917)
diff --git a/src/testdir/test_arglist.vim b/src/testdir/test_arglist.vim
index f75bad1..2a2425a 100644
--- a/src/testdir/test_arglist.vim
+++ b/src/testdir/test_arglist.vim
@@ -65,13 +65,19 @@
   %argd
   edit d
   arga
-  call assert_equal(len(argv()), 1)
-  call assert_equal(get(argv(), 0, ''), 'd')
+  call assert_equal(1, len(argv()))
+  call assert_equal('d', get(argv(), 0, ''))
+
+  %argd
+  edit some\ file
+  arga
+  call assert_equal(1, len(argv()))
+  call assert_equal('some file', get(argv(), 0, ''))
 
   %argd
   new
   arga
-  call assert_equal(len(argv()), 0)
+  call assert_equal(0, len(argv()))
 endfunc
 
 func Init_abc()