In c546a2b0 this was changed to mimic how the browser behaves in a real
situation but left out types that were registered.

When this was changed it didn't take `text/plain` or `text/html` content
types into account. This is a problem if you're manipulating the
`Content-Type` headers in your controller tests, and expect a certain
result.

The reason I changed this to use `to_sym` is because if the
`Content-Type` is not registered then the symbol will not exist. If it's
one of the special types we handle that specifically (:json, :xml, or
:url_encoded_form). If it's any registered type we handle it by setting
the `path_parameters` and then the `request_parameters`. If the `to_sym`
returns nil an error will be thrown.

If the controller test sets a `Content-Type` on the request that `Content-Type`
should remain in the header and pass along the filename.

For example:
If a test sets a content type on a post
```
@request.headers['CONTENT_TYPE'] = 'text/plain'
post :create, params: { name: 'foo.txt' }
```

Then `foo.txt` should be in the `request_parameters` and params related
to the path should be in the `path_parameters` and the `Content-Type`
header should match the one set in the `@request`. When c546a2b0 was
committed `text/plain` and `text/html` types were throwing a "Unknown
Content-Type" error which is misleading and incorrect.

Note: this does not affect how this is handled in the browser, just how
the controller tests handle setting `Content-Type`.