Added Merb.testing? method

This will check for either $TESTING (as used by merb internals/plugins) or
Merb::Config[:testing], which can be set in the config options. Having an
option like this provides a nice interface for the more public facing parts of
Merb; the auto-generated spec_helper.rb file for example can just use the test
environment setting.

lib/merb-core/test/helpers/request_helper.rb

@@ -97,6 +97,7 @@ def fake_request(env = {}, opt = {})
       #---
       # @public
       def dispatch_to(controller_klass, action, params = {}, env = {}, &blk)
+        action = action.to_s
         request_body = { :post_body => env[:post_body], :req => env[:req] }
         request = fake_request(env.merge(
           :query_string => Merb::Request.params_to_query_string(params)), request_body)