Correcting some mismatched test descriptions

The descriptions of some of the tests seemed to match other tests.

For example:

it "returns false when status is not set" do
  user = Twitter::User.new(:id => 7505382)
  expect(user.profile_banner_uri?).to be false
end

Should probably be:

it "returns false when profile_banner_url is not set" do

Since it is testing if profile_banner_url is set and not if status is set.