Fix helpers blocks failing when aliasing Base methods

The following would fail with a "undefined method `escape_html'"
exception because the block was being evaluated within a new module
and then included in Base. i.e., the following no longer worked:

    helpers { alias_method :h, :escape_html }

This was introduced just recently as part of the extensions work
foca did.

lib/sinatra/base.rb

@@ -695,7 +695,7 @@ module Sinatra
 
     public
       def helpers(*extensions, &block)
-        extensions << Module.new(&block) if block
+        class_eval(&block) if block_given?
         include *extensions
       end
 

test/helpers_test.rb

@@ -384,7 +384,7 @@ module HelperOne; def one; '1'; end; end
 module HelperTwo; def two; '2'; end; end
 
 describe 'Adding new helpers' do
-  it 'should allow passing a list of modules' do
+  it 'takes a list of modules to mix into the app' do
     mock_app {
       helpers HelperOne, HelperTwo
 
@@ -404,7 +404,7 @@ describe 'Adding new helpers' do
     assert_equal '2', body
   end
 
-  it 'should take a block and mix it into the app' do
+  it 'takes a block to mix into the app' do
     mock_app {
       helpers do
         def foo
@@ -420,4 +420,20 @@ describe 'Adding new helpers' do
     get '/'
     assert_equal 'foo', body
   end
+
+  it 'evaluates the block in class context so that methods can be aliased' do
+    mock_app {
+      helpers do
+        alias_method :h, :escape_html
+      end
+
+      get '/' do
+        h('42 < 43')
+      end
+    }
+
+    get '/'
+    assert ok?
+    assert_equal '42 &lt; 43', body
+  end
 end