Fix :provides crashes with no Accept header [#139]

An exception was raised on every request that did not have an
Accept header due to the Accept parsing code calling split on
nil.

The Sinatra::Request#accept method now returns an empty collection
if the HTTP Accept header is not present.

lib/sinatra/base.rb

@@ -12,7 +12,7 @@ module Sinatra
     end
 
     def accept
-      @env['HTTP_ACCEPT'].split(',').map { |a| a.strip }
+      @env['HTTP_ACCEPT'].to_s.split(',').map { |a| a.strip }
     end
 
     # Override Rack 0.9.x's #params implementation (see #72 in lighthouse)

test/routing_test.rb

@@ -484,4 +484,18 @@ describe "Routing" do
       assert_equal type, response.headers['Content-Type']
     end
   end
+
+  it 'degrades gracefully when optional accept header is not provided' do
+    mock_app {
+      get '/', :provides => :xml do
+        request.env['HTTP_ACCEPT']
+      end
+      get '/' do
+        'default'
+      end
+    }
+    get '/'
+    assert ok?
+    assert_equal 'default', body
+  end
 end