Recognise protocols other than "http" in _links_add_base(). Props SergeyBiryukov. Fixes #19665.

Previously "https" URLs used in plugin READMEs displayed by install_plugin_information() would have the plugin's extend URL prepended.


git-svn-id: https://core.svn.wordpress.org/trunk@20466 1a063a9b-81f0-0310-95a4-ce76da25c4cd