Support for `content` when describing parameters

Documented here: https://swagger.io/docs/specification/describing-parameters/ under `schema vs content`

Example Swagger definition that causes examples to fail with `Error: Parameter 'foo' has no schema`:

```
openapi: 3.0.1
info:
version: 1.0.0
paths:
/foo:
get:
operationId: getFoo
parameters:
- in: query
name: foo
content:
application/json:
schema:
$ref: '#/components/parameters/Foo'
required: true
responses:
204:
description: OK
components:
parameters:
Foo:
type: object
properties:
bar:
type: string
baz:
type: string
```

This would allow query strings in the format of:
```
/foo?foo={"bar":"value","baz":"value"}
```
and would deserialize that into `Foo`.