Cannot start builds on Jenkins when using a reverse proxy

Issue Summary

If the reverse proxy requires a "Content-Length" to be set on the POST request it is impossible to start a build using the triple dot menu in Bitbucket.

Steps to Reproduce

1. Set up a reverse proxy such as IIS
2. Try to trigger a build from within Bitbucket

Expected Results

Build is started

Actual Results

Build does not start an error is displayed to the user.

Workaround

There is no workaround, apart from changing configuration of the reverse proxy