Write specs of Contact model
user
```ruby
require "spec_helper"

describe Contact do
  it "is valid with a firstname, lastname and email" do
    contact = Contact.new(
                          firstname: "Aaron",
                          lastname: "Summer",
                          email: "tester@example.com"
                         )
    expect(contact).to be_valid
  end

  it "is invalid without a firstname" do
    expect(Contact.new(firstname: nil)).to have(1).errors_on(:firstname)
  end

  it "is invalid withour a lastname" do
    expect(Contact.new(lastname: nil)).to have(1).errors_on(:lastname)
  end

  it "is invalid without an email address" do
    expect(Contact.new(email: nil)).to have(1).errors_on(:email)
  end

  it "is invalid with a duplicate email address" do
    Contact.create(
                   firstname: "Joe",
                   lastname: "Tester",
                   email: "tester@example.com"
                  )
    contact = Contact.create(
                             firstname: "Jane",
                             lastname: "Tester",
                             email: "tester@example.com"
                            )
    expect(contact).to have(1).errors_on(:email)
  end

  it "returns a contact's full name as a string"
end
```
assistant