Currently, there are two ways (kind of) for accessing `params` inside a
service:
- when there is no contract or it hasn’t been reached yet, `params` is
just the hash that was provided to the service. To access a key, you
have to use the bracket notation `params[:my_key]`.
- when there is a contract and it has been executed successfully,
`params` now references the contract and the attributes are accessible
using methods (`params.my_key`).
This patch unifies how `params` exposes its attributes. Now, even if
there is no contract at all in a service, `params` will expose its
attributes through methods, that way things are more consistent.
This patch also makes sure there is always a `params` object available
even when no `params` key is provided to the service (this allows a
contract to fail because its attributes are blank instead of having the
service raising an error because it doesn’t find `params` in its context).
With the current implementation, a service step can be written as:
```ruby
def my_step(a_default_value: 2)
…
end
```
That’s a pattern we want to avoid as default values (if needed) should
be probably defined in a contract.
This patch makes a service raise an exception if a default value is
encountered.
