Fix forwarding of repeated headers

Repeated headers may not always be combined into a single header separated with a comma (eg: cookies) but they can always be repeated for the different values.

fixes ysbaddaden#32

src/prax/parser/header.cr

@@ -31,7 +31,9 @@ module Prax
       end
 
       def to_s
-        "#{name}: #{values.join(", ")}"
+        values.map {|value|
+          "#{name}: #{value}"
+        }.join("\r\n")
       end
 
       def to_i

test/proxy_test.rb

@@ -31,7 +31,7 @@ def test_supports_xip_io
 
   def test_returns_multiple_set_cookie_headers
     response = Net::HTTP.get_response(URI("http://cookies.dev:20557/"))
-    assert_equal ["first=123, second=456"], response.get_fields("Set-Cookie")
+    assert_equal ["first=123", "second=456"], response.get_fields("Set-Cookie")
   end
 
   def test_supports_bundler_with_special_gems