Partitions with custom reduce are broken

[nickva]

Don't merge just a draft PR to show the diff

make elixir tests=test/elixir/test/partition_view_test.exs:314

  * test query with custom reduce works (708.2ms)

  1) test query with custom reduce works (ViewPartitionTest)
     test/elixir/test/partition_view_test.exs:314
     Assertion with == failed
     code:  assert resp.status_code == 200
     left:  500
     right: 200
     stacktrace:
       test/elixir/test/partition_view_test.exs:319: (test)

Logs show:

[error] 2022-03-11T20:05:58.969707Z node1@127.0.0.1 <0.985.0> 9bc85227db req_err(2412564908) {{invalid_ejson,{p,<<"bar">>,[<<"field">>,<<"one">>]}},
 [{jiffy,encode,2,
         [{file,"/Users/nvatama/asf-3-main/src/jiffy/src/jiffy.erl"},
          {line,99}]},
  {couch_os_process,writejson,2,[{file,"src/couch_os_process.erl"},{line,97}]},
  {couch_os_process,handle_call,3,

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Checklist

• Code is written and works correctly
• Changes are covered by tests
• Any new configurable parameters are documented in rel/overlay/etc/default.ini
• A PR for documentation changes has been made in https://github.com/apache/couchdb-documentation