Change to __weak from __unsafe_unretained to fix crashes during unit …

[msqr]

…testing of many threaded HTTP responses in WebApiClient project unit tests. I kept getting crashes from the setHeadersForResponse:error: method, on the line that calls [http.defaultHeaders enumerateKeysAndObjectsUsingBlock:... and I noticed the __unsafe_unretained ivar. Switching to __weak resolved the crashes for me.