Using [] should give the same result than the accessor

[fonji]

Currently, this is what happens

contact = Hubspot::Contact.find(id)
contact[:firstname]
# => {"value"=>"Bruce", "versions"=>[redacted]} 
contact.firstname
# => "Bruce"

Which is already inconsistent but how about this?

contact = Hubspot::Contact.new(firstname: 'Bruce')
contact[:firstname]
# => 'Bruce'
contact.firstname
# => "Bruce"

This PR fixes this by having #[] always giving the value.

Upstream PR: HubspotCommunity#218