(defensively) only set RACK_ENV when rack.env present - don't default to ''

Author: kares

lib/warbler/templates/rack.erb

@@ -1,3 +1,5 @@
-ENV['RACK_ENV'] ||= '<%= (params = config.webxml.context_params; params['rack.env']) %>'
+<% if params = config.webxml.context_params && params['rack.env'] -%>
+ENV['RACK_ENV'] ||= '<%= params['rack.env'] %>'
+<% end -%>
 
 $LOAD_PATH.unshift $servlet_context.getRealPath('/WEB-INF') if $servlet_context