Fix Request#accept? NoMethodError

Allow Request#accept? to return false when type parameter does not
match any of the preferred types. When `preferred_type(type)` returns
nil, `include?` will throw a NoMethodError. By converting output to a
string, `include?` will return false.

lib/sinatra/base.rb

@@ -32,7 +32,7 @@ module Sinatra
     end
 
     def accept?(type)
-      preferred_type(type).include?(type)
+      preferred_type(type).to_s.include?(type)
     end
 
     def preferred_type(*types)

test/request_test.rb

@@ -89,4 +89,9 @@ class RequestTest < Test::Unit::TestCase
     assert request.accept?('text/html')
     assert_equal '*/*', request.preferred_type.to_s
   end
+
+  it 'does not accept */* when parameter does not match HTTP_ACCEPT' do
+    request = Sinatra::Request.new 'HTTP_ACCEPT' => 'application/json'
+    assert !request.accept?('text/html')
+  end
 end