set Content-Length HTTP response header

[snarfed]

...so that clients can detect and avoid responses that are too big for them to handle.

example: http://www.evdemon.org/2015/learning-more-about-quill has a >1MB inline image in the post content (ie <img src="data:image/png;base64,...>), which is also hoisted into the og:image meta tag, so the total HTML size is >2MB. found in snarfed/bridgy#525.

(not high priority, obviously. thanks in advance. hope you all are good!)