patch 8.1.1954: more functions can be used as a method

Problem:    More functions can be used as a method.
Solution:   Allow more functions to be used as a method.
diff --git a/src/testdir/test_json.vim b/src/testdir/test_json.vim
index 8f85a58..94481d9 100644
--- a/src/testdir/test_json.vim
+++ b/src/testdir/test_json.vim
@@ -70,7 +70,7 @@
 func Test_json_encode()
   call assert_equal(s:json1, json_encode(s:var1))
   call assert_equal(s:json2, json_encode(s:var2))
-  call assert_equal(s:json3, json_encode(s:var3))
+  call assert_equal(s:json3, s:var3->json_encode())
   call assert_equal(s:json4, json_encode(s:var4))
   call assert_equal(s:json5, json_encode(s:var5))
 
@@ -110,7 +110,7 @@
 func Test_json_decode()
   call assert_equal(s:var1, json_decode(s:json1))
   call assert_equal(s:var2, json_decode(s:json2))
-  call assert_equal(s:var3, json_decode(s:json3))
+  call assert_equal(s:var3, s:json3->json_decode())
   call assert_equal(s:var4, json_decode(s:json4))
   call assert_equal(s:var5, json_decode(s:json5))
 
@@ -188,7 +188,7 @@
 func Test_js_encode()
   call assert_equal(s:json1, js_encode(s:var1))
   call assert_equal(s:json2, js_encode(s:var2))
-  call assert_equal(s:json3, js_encode(s:var3))
+  call assert_equal(s:json3, s:var3->js_encode())
   call assert_equal(s:json4, js_encode(s:var4))
   call assert_equal(s:json5, js_encode(s:var5))
 
@@ -226,7 +226,7 @@
 func Test_js_decode()
   call assert_equal(s:var1, js_decode(s:json1))
   call assert_equal(s:var2, js_decode(s:json2))
-  call assert_equal(s:var3, js_decode(s:json3))
+  call assert_equal(s:var3, s:json3->js_decode())
   call assert_equal(s:var4, js_decode(s:json4))
   call assert_equal(s:var5, js_decode(s:json5))