patch 7.4.1450

Problem:    Json encoding still fails when encoding is not utf-8.
Solution:   Set 'encoding' before :scriptencoding.  Run the json test
            separately to avoid affecting other tests.

src/testdir/test_json.vim

@@ -5,8 +5,8 @@
 if !has('multi_byte')
   finish
 endif
-scriptencoding utf-8
 set encoding=utf-8
+scriptencoding utf-8
 
 let s:json1 = '"str\"in\\g"'
 let s:var1 = "str\"in\\g"