Underscores not converted to spaces by mbfl_mime_header_decode

[Pudge601]

This issue was raised as a bug in PHP with the mb_decode_mimeheader function: https://bugs.php.net/bug.php?id=68821, and in the comments of that bug report, it was stated that it is a problem which needs to be fixed in this library.

However, it doesn't appear that any issue was created for this library to get the bug fixed, so I'm creating this one.

This is the sample input, and expected vs actual output taken from the bug report I've linked;

Sample input:

Subject: =?ISO-8859-1?Q?spring_into_ALDI_for_this_week=92s_special_buys_fr?=
 =?ISO-8859-1?Q?om_thurs_30_sep_=96_hurry,_limited_stocks_in_store?=

Expected result:

spring into ALDI for this weeks special buys from thurs 30 sep  hurry, limited stocks in store

Actual result:

spring_into_ALDI_for_this_weeks_special_buys_from_thurs_30_sep__hurry,_limited_stocks_in_store

[bohwaz]

Ah thanks I was just about to report that issue here :)