/write endpoint inconsistent content-type under error condition

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test case:

when POSTing the invalid line protocol to the `/write?db=test` endpoint:

```
ha ha ha
```

Expected result:
content-type: text/plain
```
unable to parse 'ha ha ha': invalid field format
```

OR

content-type: application/json
```
{
"error:" "unable to parse 'ha ha ha': invalid field format"
}
```

Actual result (bug):
content-type: application/json
```
unable to parse 'ha ha ha': invalid field format
```

Start by reproducing the invalid POST to the `/write?db=test` endpoint with `ha ha ha`, then trace the error response handling for that endpoint. Done means the response uses a consistent content type and matching body format, either plain text or the specified JSON structure.

Written by the indexing model from the issue text.