patch 9.1.0329: String interpolation fails for Dict type

Problem:  String interpolation fails for Dict type
Solution: Support Dict data type properly, also support :put =Dict
          (without having to convert it to string() first)
          (Yegappan Lakshmanan)

fixes: #14529
closes: #14541

Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_expr.vim b/src/testdir/test_expr.vim
index c1869c1..1fa460d 100644
--- a/src/testdir/test_expr.vim
+++ b/src/testdir/test_expr.vim
@@ -950,6 +950,10 @@
     endif
     call assert_equal(0, tmp)
 
+    #" Dict interpolation
+    VAR d = {'a': 10, 'b': [1, 2]}
+    call assert_equal("{'a': 10, 'b': [1, 2]}", $'{d}')
+
     #" Stray closing brace.
     call assert_fails('echo $"moo}"', 'E1278:')
     #" Undefined variable in expansion.