Add test case from RFC2047 and fix last pattern

See: https://www.ietf.org/rfc/rfc2047#section-8
And fix (=?ISO-8859-1?Q?a?= =?ISO-8859-2?Q?_b?=) as (a b).

Author: youkidearitai

ext/mbstring/mbstring.c

@@ -6402,6 +6402,8 @@ static zend_string* mb_mime_header_decode(zend_string *input, const mbfl_encodin
 						/* We will only actually output a space if this is not immediately followed
 						 * by another valid encoded word */
 						space_pending = true;
+					} else {
+						space_pending = false;
 					}
 					continue;
 				}

ext/mbstring/tests/gh20674.phpt

@@ -17,9 +17,24 @@ var_dump(mb_decode_mimeheader($subject));
 
 $subject = "=?us-ascii?Q?The_PH?= \t =?us-ascii?Q?P_8.5?=";
 var_dump(mb_decode_mimeheader($subject));
+
+// from RFC 2047 https://www.ietf.org/rfc/rfc2047
+var_dump(mb_decode_mimeheader("(=?ISO-8859-1?Q?a?= =?ISO-8859-2?Q?_b?=)"));
+var_dump(mb_decode_mimeheader("(=?ISO-8859-1?Q?a_b?=)"));
+var_dump(mb_decode_mimeheader("(=?ISO-8859-1?Q?a?=  =?ISO-8859-1?Q?b?=)"));
+var_dump(mb_decode_mimeheader("(=?ISO-8859-1?Q?a?= b)"));
+var_dump(mb_decode_mimeheader("(=?ISO-8859-1?Q?a?=  =?ISO-8859-1?Q?b?=)"));
+var_dump(mb_decode_mimeheader("(=?ISO-8859-1?Q?a?=
+	=?ISO-8859-1?Q?b?=)"));
 ?>
 --EXPECTF--
 string(11) "The PHP 8.5"
 string(11) "The PHP 8.5"
 string(11) "The PHP 8.5"
 string(11) "The PHP 8.5"
+string(5) "(a b)"
+string(5) "(a b)"
+string(4) "(ab)"
+string(5) "(a b)"
+string(4) "(ab)"
+string(4) "(ab)"