patch 8.2.1378: cannot put space between function name and paren

Problem:    Cannot put space between function name and paren.
Solution:   Allow this for backwards compatibility.

src/testdir/test_expr.vim

@@ -599,6 +599,11 @@ func Test_expr_eval_error()
   call assert_fails("let v = -{}", 'E728:')
 endfunc
 
+func Test_white_in_function_call()
+  let text = substitute ( 'some text' , 't' , 'T' , 'g' )
+  call assert_equal('some TexT', text)
+endfunc
+
 " Test for float value comparison
 func Test_float_compare()
   CheckFeature float