Added to Innsights::Report the ability to overwrite the user group configuration, with nil

[bhserna]

Innsights config

Innsights.setup do
  user :User do
    group :company
  end
end

Before

report = Innsights::Report.new("Mention", user: user)
report.group.object.should eq user.company

report = Innsights::Report.new("Mention", user: user, group: nil)
report.group.object.should eq user.company

After

report = Innsights::Report.new("Mention", user: user)
report.group.object.should eq user.company

report = Innsights::Report.new("Mention", user: user, group: nil)
report.group.should eq nil