Updating the Content-Length header.

Hi Max,

This pull request fixes an issue our clients have been experiencing the last few days. WordPress sites which sit behind Cloudflare and have this plugin active experience drastic performance degradation because the `Content-Length` header doesn't match the size of the response body *after* the trim.

Cloudflare will sit there and wait for the rest of the non-existant body to arrive until they timeout, which in my testing has taken a minute or so for "bypassed" A records and 3 minutes for "accelerated" A records.