codewriter: Don't use string.replace() to apply header_to_override
codewriter: Don't use string.replace() to apply header_to_override

The underlying regex causes faulty replacements.

https://bugzilla.gnome.org/show_bug.cgi?id=731322

GNOME/vala,frida/vala,frida/vala,frida/vala,frida/vala,GNOME/vala,GNOME/vala,GNOME/vala,frida/vala