Fix header_str in parallel fetches

Hi guys,

We had a problem when using newrelic_rpm 3.6.6.147 with Feedzirra.
It works with 3.6.5.130.

Our code is like this

``` Ruby
Feedzirra::fetch_and_parse(url).entries.each do
	#Code
end
```

We pass the correct url but result of fetch_and_parse method is always nil. This is the reason

* header_str method of Curl::Easy class always returns nil in parallel fetches in 3.6.6.147. because, install_header_callback method is not called.

I think it should be returns original header_str in this case. I fixed the code like that.
How do you think? or did I do something wrong?