Add option to turn off X-Content-Digest header

* Strips the header from the cache response so
  that downstream apps don't see it.

lib/rack/cache/context.rb

@@ -265,6 +265,10 @@ def fetch
     def store(response)
       strip_ignore_headers(response)
       metastore.store(@request, response, entitystore)
+      # Remove header so downstream apps won't choke on it,
+      # but only if explicitly disabled. Can't be stripped
+      # untile the digest is used to store the value
+      response.headers.delete('X-Content-Digest') if disable_digest_header?
       response.headers['Age'] = response.age.to_s
     rescue Exception => e
       log_error(e)

lib/rack/cache/options.rb

@@ -81,7 +81,8 @@ def option_name(key)
     # Set of response headers that are removed before storing them in the
     # cache. These headers are only removed for cacheable responses.  For
     # example, in most cases, it makes sense to prevent cookies from being
-    # stored in the cache.
+    # stored in the cache. It may also be useful to ignore X-Content-Digest
+    # if Rack::Cache is talking to another app that sets it.
     #
     # Default: ['Set-Cookie']
     option_accessor :ignore_headers
@@ -109,6 +110,9 @@ def option_name(key)
     # be used.
     option_accessor :use_native_ttl
 
+    # Specifies whether Rack::Cache should return an X-Content-Digest header
+    option_accessor :disable_digest_header
+
     # The underlying options Hash. During initialization (or outside of a
     # request), this is a default values Hash. During a request, this is the
     # Rack environment Hash. The default values Hash is merged in underneath
@@ -151,6 +155,7 @@ def initialize_options(options={})
         'rack-cache.allow_reload'     => false,
         'rack-cache.allow_revalidate' => false,
         'rack-cache.use_native_ttl'   => false,
+        'rack-cache.disable_digest_header' => false,
       }
       self.options = options
     end

test/context_test.rb

@@ -25,6 +25,19 @@
     response.headers.should.not.include 'Age'
   end
 
+  it 'does not respond with x-content-digest if disabled' do
+    respond_with 200, 'Cache-Control' => 'public', 'ETag' => '"FOO"'
+    get '/', 'rack-cache.disable_digest_header' => true
+
+    response.should.be.ok
+    response.body.should.equal 'Hello World'
+    response.headers.should.include 'Date'
+    response['X-Content-Digest'].should.be.nil
+    cache.trace.should.include :miss
+    cache.trace.should.include :store
+    cache.metastore.to_hash.keys.length.should.equal 1
+  end
+
   %w[post put delete].each do |request_method|
     it "invalidates on #{request_method} requests" do
       respond_with 200
@@ -452,6 +465,7 @@
     cache.metastore.to_hash.keys.length.should.equal 1
   end
 
+
   it 'caches responses with a max-age directive' do
     respond_with 200, 'Cache-Control' => 'max-age=5'
     get '/'