libcurl ignores headers with empty value, leading to signature mismatches
If you add a header with no content as in 'Accept:' (no data on the right side of the colon), the internally used header will get disabled. With this option you can add new headers, replace internal headers and remove internal headers. To add a header with no content (nothing to the right side of the colon), use the form 'MyHeader;' (note the ending semicolon).
This shows up in multisite when requests are forwarded to the master zone. If the original request has a header in x_meta_map with an empty value, it's included in our signature but isn't actually sent over the wire. The receiver is then unable to calculate the correct signature.