g_mime_object_write_to_stream() adds an extra space in the References: header after the colon. There is no extra space in the value returned by g_mime_object_get_header().

This causes the notmuch [1] test suite to fail with diff from expected to actual result:

        -References: <msg-001@notmuch-test-suite>
        +References:  <msg-001@notmuch-test-suite>

This used to work, and looking into gmime sources, the likely culprit seems to be
commit a248cc044c6ad55505939363aa858c930867a014
Author: Jeffrey Stedfast <fejj@gnome.org>
Date:   Sun Sep 29 13:23:30 2013 -0400

    Implemented custom header writer for References

introduced in gmime 2.6.19.

Indeed, write_references() has first:

	g_string_append_len (folded, ": ", 2);

and then within the while loop for the non-folding branch before adding any values:

	g_string_append_c (folded, ' ');

[1] http://notmuchmail.org/