Add logging / error handling to GovukTimezone

The more I think about it, the more I worry that having the gem forcibly
set time_zone to London without any warnings could lead to a confusing /
hard to debug problem down the line.

For example, someone trying to set config.time_zone explicitly to UTC,
or trying to set it to something else entirely, and then being confused
when it's still London.

There's no way I can think of to distinguish the "time_zone is UTC
because that's the default" situation from the "time_zone is UTC because
it's been set explicitly" situation. Logging an info message at least
increases the chance that someone will notice what's going on if they
are trying to set it to UTC.

We're also logging the situation where time_zone is set to London - this
config is fairly widespread, and it will become redundant following this
change to the gem. However it's harmless, so I don't think it's worth
chasing up all the other edge cases.

All other values of time_zone should be considered errors, as it would
be very confusing to have them silently overridden.

lib/govuk_app_config/govuk_timezone.rb

@@ -1,5 +1,14 @@
 module GovukTimezone
   def self.configure(config)
+    case config.time_zone
+    when "UTC"
+      Rails.logger.info "govuk_app_config changing time_zone from UTC (the default) to London"
+    when "London"
+      Rails.logger.info "govuk_app_config always sets time_zone to London - there is no need to set config.time_zone in your app"
+    else
+      raise "govuk_app_config prevents configuring time_zones other than London - config.time_zone was set to #{config.time_zone}"
+    end
+
     config.time_zone = "London"
   end
 end

spec/lib/govuk_timezone_spec.rb

@@ -3,10 +3,30 @@
 
 RSpec.describe GovukError do
   describe ".configure" do
-    it "should set time_zone to London" do
-      config = double(:config)
-      expect(config).to receive(:time_zone=).with("London")
+    let(:config) { Rails::Railtie::Configuration.new }
+    let(:logger) { instance_double("ActiveSupport::Logger") }
+
+    before do
+      allow(Rails).to receive(:logger).and_return(logger)
+    end
+
+    it "should override the default UTC time_zone to London" do
+      config.time_zone = "UTC"
+      expect(logger).to receive(:info)
+      GovukTimezone.configure(config)
+      expect(config.time_zone).to eq("London")
+    end
+
+    it "should leave time_zones set to London as London" do
+      config.time_zone = "London"
+      expect(logger).to receive(:info)
       GovukTimezone.configure(config)
+      expect(config.time_zone).to eq("London")
+    end
+
+    it "should raise an error if configured with any other time zone" do
+      config.time_zone = "Shanghai"
+      expect { GovukTimezone.configure(config) }.to raise_error(/govuk_app_config prevents configuring time_zones/)
     end
   end
 end