TemplateRunner environment method should override defaults

If you run
rails newapp -m template.rb

where template.rb contains:
environment "config.time_zone = 'Rome'"

the generated config/environment.rb contains two assignments of
time_zone.

In http://rails.lighthouseapp.com/projects/8994/tickets/1663 I have
uploaded a patch to make TemplateRunner override default values in
environment.rb.