patch 8.2.1361: error for white space after expression in assignment

Problem:    Error for white space after expression in assignment.
Solution:   Skip over white space. (closes #6617)
diff --git a/src/testdir/test_expr.vim b/src/testdir/test_expr.vim
index b131543..b4eeea6 100644
--- a/src/testdir/test_expr.vim
+++ b/src/testdir/test_expr.vim
@@ -55,6 +55,9 @@
   let d['a'] = 'aaa'
   call assert_equal('none', d[''])
   call assert_equal('aaa', d['a'])
+
+  let d[ 'b' ] = 'bbb'
+  call assert_equal('bbb', d[ 'b' ])
 endfunc
 
 func Test_strgetchar()