bug: string expected to convert to in dag-json tests

[SgtPooki]

cc @lidel

This CID references some text: I am a txt file on path with utf8

Why are we trying to confirm that the content-type is dag-json? shouldn't this be failing...

more details in ipfs/helia-verified-fetch#35

Gateway conformance output:

[gateway-conformance] �=== RUN   TestDagPbConversion/GET_UnixFS_file_as_DAG-JSON_with_format=dag-json_converts_to_the_expected_Content-Type/Status_code
[gateway-conformance] �--- PASS: TestDagPbConversion/GET_UnixFS_file_as_DAG-JSON_with_format=dag-json_converts_to_the_expected_Content-Type/Status_code (0.00s)
[gateway-conformance] �=== NAME  TestDagPbConversion/GET_UnixFS_file_as_DAG-JSON_with_format=dag-json_converts_to_the_expected_Content-Type
[gateway-conformance] �=== RUN   TestDagPbConversion/GET_UnixFS_file_as_DAG-JSON_with_format=dag-json_converts_to_the_expected_Content-Type/Header_Content-Type
[gateway-conformance] 
[gateway-conformance]     run.go:53: 
[gateway-conformance]         Name: GET UnixFS file as DAG-JSON with format=dag-json converts to the expected Content-Type
[gateway-conformance]         Hint: 
[gateway-conformance]         
[gateway-conformance]         Error: Header 'Content-Type' expected 'application/vnd.ipld.dag-json', got 'text/plain; charset=utf-8'
[gateway-conformance]         
[gateway-conformance]         Expected Request:
[gateway-conformance]         {
[gateway-conformance]           "method": "GET",
[gateway-conformance]           "path": "/ipfs/bafkreialihlqnf5uwo4byh4n3cmwlntwqzxxs2fg5vanqdi3d7tb2l5xkm",
[gateway-conformance]           "query": {
[gateway-conformance]             "format": [
[gateway-conformance]               "dag-json"
[gateway-conformance]             ]
[gateway-conformance]           }
[gateway-conformance]         }
[gateway-conformance]         
[gateway-conformance]         Actual Request:
[gateway-conformance]         GET /ipfs/bafkreialihlqnf5uwo4byh4n3cmwlntwqzxxs2fg5vanqdi3d7tb2l5xkm?format=dag-json HTTP/1.1
[gateway-conformance]         Host: host.docker.internal:8080
[gateway-conformance]         User-Agent: Go-http-client/1.1
[gateway-conformance]         Accept-Encoding: gzip
[gateway-conformance]         
[gateway-conformance]         
[gateway-conformance]         
[gateway-conformance]         Expected Response:
[gateway-conformance]         {
[gateway-conformance]           "statusCode": 200,
[gateway-conformance]           "headers": [
[gateway-conformance]             {
[gateway-conformance]               "key": "Content-Type",
[gateway-conformance]               "check": {}
[gateway-conformance]             },
[gateway-conformance]             {
[gateway-conformance]               "key": "Content-Disposition",
[gateway-conformance]               "check": {}
[gateway-conformance]             },
[gateway-conformance]             {
[gateway-conformance]               "key": "Content-Type",
[gateway-conformance]               "check": {},
[gateway-conformance]               "not": true
[gateway-conformance]             }
[gateway-conformance]           ],
[gateway-conformance]           "body": "eyIvIjp7ImJ5dGVzIjoiU1NCaGJTQmhJSFI0ZENCbWFXeGxJRzl1SUhCaGRHZ2dkMmwwYUNCMWRHWTRDZyJ9fQ=="
[gateway-conformance]         }
[gateway-conformance]         
[gateway-conformance]         Actual Response:
[gateway-conformance]         HTTP/1.1 200 OK
[gateway-conformance]         Content-Length: 31
[gateway-conformance]         Access-Control-Allow-Headers: Content-Type, Range, User-Agent, X-Requested-With
[gateway-conformance]         Access-Control-Allow-Methods: GET, HEAD, OPTIONS
[gateway-conformance]         Access-Control-Allow-Origin: *
[gateway-conformance]         Access-Control-Expose-Headers: Content-Range, Content-Length, X-Ipfs-Path, X-Ipfs-Roots, X-Chunked-Output, X-Stream-Output
[gateway-conformance]         Connection: keep-alive
[gateway-conformance]         Content-Type: text/plain; charset=utf-8
[gateway-conformance]         Date: Fri, 29 Mar 2024 23:56:49 GMT
[gateway-conformance]         Keep-Alive: timeout=72