Add requestBody component

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Hello there.

I am trying to create a path operation using ApiSpec as follows:
**apiSpec.path(
path = "/my-item",
operations = dict(
post = dict(
tags = ["MyItem"],
operationId = "createMyItem",
description = "Create MyItem",
requestBody ={ "$ref": "#/components/requestBodies/MyItem" } ,
responses = {'200': {'$ref': '#/components/responses/OK'}, '500': {'$ref': '#/components/responses/GeneralError'}}
)
))**

When I try to pass the ResponseBody as ref, as above, I get below error:

**File "c:\workspace\.venv\Lib\site-packages\apispec\core.py", line 549, in path
self.components.resolve_refs_in_path(self._paths[path])
File "c:\workspace\.venv\Lib\site-packages\apispec\core.py", line 410, in resolve_refs_in_path
self._resolve_refs_in_operation(path[method])
File "c:\workspace\.venv\Lib\site-packages\apispec\core.py", line 382, in _resolve_refs_in_operation
self._resolve_refs_in_request_body(operation["requestBody"])
File "c:\workspace\.venv\Lib\site-packages\apispec\core.py", line 352, in _resolve_refs_in_request_body
for media_type in request_body["content"].values():
~~~~~~~~~~~~^^^^^^^^^^^
KeyError: 'content'**

I could pass the whole ResponseBody schema, which is already present in my apiSpec.options['components']['requestBodies']['MyItem'], but this would add duplication for each of my path operations and I would like to avoid this.

Could you advice if it is possible to pass a $ref for RequestBody, which should result in:

_post:
description: Create MyItem
operationId: createMyItem
requestBody:
$ref: '#/components/requestBodies/MyItem'_

In my generated .yaml file?

Also some doc with more real case example would be very handy to work with the ApiSpec library. I checked examples in the [ApiSpec read the docs](https://apispec.readthedocs.io/en/latest/index.html#) but I could not find proper example of how to add / manipulate operation to a path etc.

Thank you in advance :)

Start in apispec/core.py at _resolve_refs_in_operation and _resolve_refs_in_request_body, using the reported KeyError as the entry point. Compare the generated specification with the requested requestBody $ref output, then review the linked documentation examples for operation and path usage. Done means references work without duplication and the relevant usage is documented.

Written by the indexing model from the issue text.