[OCaml] Fix enum returnType in of_json.mustache for Api Impl

[syntakker]

@cgensoul

Enum return types seem to be incorrectly handled in api implementations *_api.ml for json swagger.txt.

For operations returning enums, processing of return values is rendered as

    Request.read_json_body_as_list_of (JsonSupport.unwrap Push_result.of_yojson) resp body

where PushResult is an enum. It should however render to

    Request.read_json_body_as_list_of (JsonSupport.unwrap Enums.pushresult_of_yojson) resp body

The cause seems to be that neither isEnum nor datatypeWithEnum is available for operations in of_json.mustache:

{{#isEnum}}JsonSupport.unwrap Enums.{{{datatypeWithEnum}}}_of_yojson{{/isEnum}}{{^isEnum}} ...

This fix works for me, however I suppose that a solution could be substantially more elegant if I only was more with conversant CodeGen internals.

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[wing328]

Tested locally and the code compiles without issues whereas without this fix, I got errors building the client, e.g.

# (cd _build/default && /Users/williamcheng/.opam/default/bin/ocamlopt.opt -w -40 -w -27 -g -I .petstore_client.objs/byte -I .petstore_client.objs/native -I /Users/williamcheng/.opam/default/lib/astring -I /Users/williamcheng/.opam/default/lib/base -I /Users/williamcheng/.opam/default/lib/base/caml -I /Users/williamcheng/.opam/default/lib/base/shadow_stdlib -I /Users/williamcheng/.opam/default/[...]
# File "src/apis/pet_api.ml", line 56, characters 50-69:
# 56 |     Request.read_json_body_as (JsonSupport.unwrap Enum_test.of_yojson) resp body
#                                                        ^^^^^^^^^^^^^^^^^^^
# Error: Unbound module Enum_test

[wing328]

@syntakker thanks for the PR, which has been included in the v5.1.0 release: https://twitter.com/oas_generator/status/1373636654024380423