Allow custom headers in XML-RPC request

[ericmann]

WordPress avails an override to the default IXR_Client object that uses its own WP_Http wrapper for making the request. The wrapper itself is pluggable, but there are no filters before the call to wp_remote_post that allow developers to intercept and modify the request.

As a result, sending custom headers with the request (like cookies) requires developers in turn override IXR_Client with their own implementation in order to add the custom data. One example, which specifically addresses sending cookies with the request, is ​https://gist.github.com/ericmann/5808805.

Instead, the built-in WP_HTTP_IXR_Client object should pass its header array through a WordPress filter before building the remote request. Developers can hook in to this filter to add their own custom headers - be they cookies, authentication, language, or whatever.

[ericmann]

Patch adding a filter to request headers.

[ericmann]

The patch adds brackets to the existing single-line conditional statements in the file as the indentation can be tricky to follow and what the code is doing here should be absolutely clear.

[danhgilmore]

I had this requirement (to send cookies in my request) and Eric offered to help. This was a simple and quick fix, and exactly what I needed. I've already got it working on one installation and I'll have it on my other two installations by COB.

[ericmann]

Updated patch file.

[ericmann]

Updated patch file against the new development repository :-)

[chriscct7]

Looks good

[wonderboymusic]

Filter docs

[ericmann]

Filter docs

[ericmann]

Added a docblock for the filter.

[wonderboymusic]

In 34164:

Add a filter to allow custom headers to be sent via the WP HTTP IXR client: 'wp_http_ixr_client_headers'.

Props ericmann.
Fixes #24598.

[DrewAPicture]

Instead of describing what the filter allows, let's describe what's filterable.

[DrewAPicture]

In 34203:

Docs: Update the hook doc summary for the wp_http_ixr_client_headers filter, introduced in [34164].

The idea is to try to reflect what the filterable value is rather than why it's filterable.

Fixes #24598.