patch 8.2.1386: backslash not removed afer space with space in 'isfname'

Problem:    Backslash not removed afer space in option with space in
            'isfname'.
Solution:   Do remove backslash before space, also when it is in 'isfname'.
            (Yasuhiro Matsumoto, closes #6651)
diff --git a/src/testdir/test_options.vim b/src/testdir/test_options.vim
index 8f27d9c..b3e9da7 100644
--- a/src/testdir/test_options.vim
+++ b/src/testdir/test_options.vim
@@ -973,4 +973,13 @@
   set winwidth&
 endfunc
 
+" Test for setting option value containing spaces with isfname+=32
+func Test_isfname_with_options()
+  set isfname+=32
+  setlocal keywordprg=:term\ help.exe
+  call assert_equal(':term help.exe', &keywordprg)
+  set isfname&
+  setlocal keywordprg&
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab