fix get http_header on use bearer token

Hello, bdanchilla.
I want to make the twitteroauth.php corresponding to the Application-only auth.
But, you had to create a great code already.
Now, I have been using your code.
When you use the BearerToken, TwitterOAuth object does not come back with the possession http_header.
I wanted a x_rate_limit_remaining in the http_header.
Therefore, I was returned to the original position of curl_setopt ().

Your code to be merged soon.