Add fallback for Accept header.

CloudFront does not appear to forward Accept headers. Without an Accept header, camo sends `Accept: undefined`.

I ran into an issue with this when requesting an image from an IIS server:

```
curl -v --header 'Accept: undefined' http://images.anandtech.com/doci/6673/OpenMoboAMD30_575px.png
* About to connect() to images.anandtech.com port 80 (#0)
*   Trying 199.19.80.14...
* connected
* Connected to images.anandtech.com (199.19.80.14) port 80 (#0)
> GET /doci/6673/OpenMoboAMD30_575px.png HTTP/1.1
> User-Agent: curl/7.24.0 (x86_64-apple-darwin12.0) libcurl/7.24.0 OpenSSL/0.9.8r zlib/1.2.5
> Host: images.anandtech.com
> Accept: undefined
> 
< HTTP/1.1 406 Not Acceptable
< Content-Type: text/html
< Server: Microsoft-IIS/7.5
< X-Powered-By: ASP.NET
< Date: Sat, 19 Jan 2013 10:27:15 GMT
< Content-Length: 1346
< 
```