Issue #3 JdkRequest now handles duplicate headers

The test works fine as it is; I discovered the bug when I tried changing the type to `ApacheRequest` and it passed.

The difference is that `setRequestProperty` overrides the existing header while `addRequestProperty` appends to it. See [javadoc for URLConnection](http://docs.oracle.com/javase/7/docs/api/java/net/URLConnection.html#setRequestProperty(java.lang.String, java.lang.String)).