Support merging of resource-default and request-specific :params

Sometimes we need to provide default request params which are relevant to all calls to REST API endpoints (i.e. client_id). However, each individual API endpoint requires its own params. These pull request allows to replace the following code:

```ruby
  resource = RestClient::Resource.new('http://some_host.com/cities')
  resource['all'].get(params: {client_id: 'some_client_id'})
  resource['show/1'].get(params: {lat: 56.83, lon: 60.58, client_id: 'some_client_id'})
```

with the following one:

```ruby
  resource = RestClient::Resource.new('http://some_host.com/cities', headers: {params: {client_id: 'some_client_id'}})
  resource['all'].get
  resource['show/1'].get(params: {lat: 56.83, lon: 60.58})
```