Treat missing User Agent like an empty User Agent. Fixes #164.

lib/sinatra/base.rb

@@ -990,7 +990,7 @@ module Sinatra
       # Will set params[:agent].
       def user_agent(pattern)
         condition do
-          if request.user_agent =~ pattern
+          if request.user_agent.to_s =~ pattern
             @params[:agent] = $~[1..-1]
             true
           else

test/routing_test.rb

@@ -611,6 +611,18 @@ class RoutingTest < Test::Unit::TestCase
     assert_equal 'Hello World', body
   end
 
+  it "treats missing user agent like an empty string" do
+    mock_app do
+      user_agent(/.*/)
+      get '/' do
+        "Hello World"
+      end
+    end
+    get '/'
+    assert_equal 200, status
+    assert_equal 'Hello World', body
+  end
+
   it "makes captures in user agent pattern available in params[:agent]" do
     mock_app {
       user_agent(/Foo (.*)/)