make use mime_type returns a string or nil

lib/sinatra/base.rb

@@ -1255,7 +1255,8 @@ module Sinatra
 
       # Lookup or register a mime type in Rack's mime registry.
       def mime_type(type, value = nil)
-        return type if type.nil? || type.to_s.include?('/')
+        return type      if type.nil?
+        return type.to_s if type.to_s.include?('/')
         type = ".#{type}" unless type.to_s[0] == ?.
         return Rack::Mime.mime_type(type, nil) unless value
         Rack::Mime::MIME_TYPES[type] = value

test/helpers_test.rb

@@ -574,6 +574,12 @@ class HelpersTest < Test::Unit::TestCase
     it 'returns the argument when given a media type string' do
       assert_equal 'text/plain', mime_type('text/plain')
     end
+
+    it 'turns AcceptEntry into String' do
+      type = mime_type(Sinatra::Request::AcceptEntry.new('text/plain'))
+      assert_equal String, type.class
+      assert_equal 'text/plain', type
+    end
   end
 
   test 'Base.mime_type registers mime type' do