Mock HttpContext.Current.Request.Headers. This is ugly but it works. credit: http://bigjimindc.blogspot.co.uk/2007/07/ms-kb928365-aspnet-requestheadersadd.html

gistfile1.txt

HttpContext.Current = new HttpContext(
    new HttpRequest("", "http://tempuri.org", ""), new HttpResponse(new StringWriter()));

NameValueCollection headers = HttpContext.Current.Request.Headers;

Type t = headers.GetType();
const BindingFlags nonPublicInstanceMethod = BindingFlags.InvokeMethod | BindingFlags.NonPublic | BindingFlags.Instance;

t.InvokeMember("MakeReadWrite", nonPublicInstanceMethod, null, headers, null);
t.InvokeMember("InvalidateCachedArrays", nonPublicInstanceMethod, null, headers, null);

// eg. add Basic Authorization header
t.InvokeMember("BaseRemove", nonPublicInstanceMethod, null, headers, new object[] { "Authorization" });
t.InvokeMember("BaseAdd", nonPublicInstanceMethod, null, headers, 
    new object[] { "Authorization", new ArrayList{"Basic " + api_key} });

t.InvokeMember("MakeReadOnly", nonPublicInstanceMethod, null, headers, null);