Remove trailing white space from json formatter

[masaori335]

Fix httpbin autest.

       file /var/tmp/ausb-6370.12163/httpbin/_output/3-tr-Default/stream.stdout.txt : Checking that /var/tmp/ausb-6370.12163/httpbin/_output/3-tr-Default/stream.stdout.txt matches gold/httpbin_3_stdout.gold - Failed
          Reason: File differences
           Gold File : /var/jenkins/workspace/autest-github/src/tests/gold_tests/h2/gold/httpbin_3_stdout.gold
           Data File : /var/tmp/ausb-6370.12163/httpbin/_output/3-tr-Default/stream.stdout.txt
             {
               "files": {}, 
               "origin": "127.0.0.1", 
               "form": {
                 "key": "value"
           -   },
           +   }, 
           ?     +
           
               "url": "http://127.0.0.1:61743/post", 
               "args": {}, 
               "headers": {
                 "Content-Length": "9", 
                 "Client-Ip": "127.0.0.1", 
                 "Accept": "*/*", 
                 "User-Agent": "curl/7.65.0-DEV", 
                 "Host": "127.0.0.1:61743", 
                 "Expect": "100-continue", 
                 "Content-Type": "application/x-www-form-urlencoded"
               }, 
               "json": null, 
               "data": ""
             }

[shukitchan]

[approve ci autest]

[SolidWallOfCode]

Yes, that was annoying.

[zwoop]

This is good. We should investigate if we can avoid spawning a new Python here, since the version can be different. A fork() would be better if possible ?

[masaori335]

I agree we should avoid spawning a new Python if possible. But the issue looks a bit general in AuTest. Let's merge this and fix them in other PRs. (Filed as #6387, #6386)

[zwoop]

Cherry-picked to v9.0.x branch.