Add support for nullable option in RequestParam annotation

I have a use case where I have validation requirements on `RequestParam` but still allow the parameter not to be set in requests (through the `nullable=true` Annotation option). The problem is that the API documentation generated show these as required parameters.
Thia feature is already supported in NelmioApiDocBundle on `QueryParam`, but not `RequestParam`, though both annotations supports the option.

My usage is like this:
```
@RequestParam(name="company", requirements=".{0,255}", description="Optional Company name on the receipt (max 255 char)", nullable=true)
```

I think it would be nice if the documentation reflected that the parameter is indeed not required. 
The issue is also related to #360, but supported through the right property (`nullable`) in RequestParam annotation.