Normalize headers from HttpStreamingResponse

Since HttpStreamingResponse#headers returns a `Hash` with `Array`-keys,
e.g.

```ruby
 {"cache-control"=>["max-age=300, public"]}
```

we need to normalise this to something like

```ruby
	{"cache-control"=> "max-age=300, public"}
```

in order to **not break other middlewares** processing the headers
further down the pipe.

In our case we encountered a problem when working with `rack-wwwhisper`,
see `Rack::WWWhisper::NoPublicCache#call` (https://github.com/wrr/rack-wwwhisper),
since it is calling `gsub` on the 'Cache-Control' header.

Author: janraasch

lib/rack/reverse_proxy.rb

@@ -82,12 +82,12 @@ def proxy(env, source_request, matcher)
       target_response.use_ssl = "https" == uri.scheme
 
       # Let rack set the transfer-encoding header
-      response_headers = format_headers(target_response.headers)
+      response_headers = Rack::Utils::HeaderHash.new Proxy.normalize_headers(format_headers(target_response.headers))
       response_headers.delete('Transfer-Encoding')
 
       # Replace the location header with the proxy domain
       if response_headers['Location'] && options[:replace_response_host]
-        response_location = URI(response_headers['Location'][0])
+        response_location = URI(response_headers['location'])
         response_location.host = source_request.host
         response_location.port = source_request.port
         response_headers['Location'] = response_location.to_s

spec/rack/reverse_proxy_spec.rb

@@ -31,6 +31,19 @@ def app
       last_response.body.should == "Proxied App"
     end
 
+    it "should produce a response header of type HeaderHash" do
+      stub_request(:get, 'http://example.com/test')
+      get '/test'
+      last_response.headers.should be_an_instance_of Rack::Utils::HeaderHash
+    end
+
+    it "should parse the headers as a Hash with values of type String" do
+      stub_request(:get, 'http://example.com/test').to_return({:headers => {'cache-control'=> 'max-age=300, public'} })
+      get '/test'
+      last_response.headers['cache-control'].should be_an_instance_of String
+      last_response.headers['cache-control'].should == 'max-age=300, public'
+    end
+
     it "should proxy requests to a lambda url when a pattern is matched" do
       stub_request(:get, 'http://example.com/2test').to_return({:body => "Proxied App2"})
       get '/2test'
@@ -166,7 +179,6 @@ def app
       it "should replace the location response header" do
         stub_request(:get, "http://example.com/test/stuff").to_return(:headers => {"location" => "http://test.com/bar"})
         get 'http://example.com/test/stuff'
-        # puts last_response.headers.inspect
         last_response.headers['location'].should == "http://example.com/bar"
       end