use preg_replace_callback instead of '/e' flag when encoding quoated printable ('/e' regex flag deprecated in PHP5.5)

replace boolean conditional checks on possibly uninitialized vars with \!empty/empty/isset as appropriate

This applies
https://github.com/simplepie/simplepie/commit/16160017bf904538c3fe4cacadc1aba19d445106
to properly handle a missing DOMDocument extention. DOMDocument is still
needed to properly parse RSS feeds, but the process will no longer fail
fatally.

not a quoted string

The used boundary should have been correct but might throw off MTAs not
expecting more than two dashes at the start. This is just a wild guess
and may not fix anything (but shouldn't break anything either).

Fixes FS#2837