* fix header regex

lib/Text/Dokuwiki/Regexps.pm

@@ -14,7 +14,7 @@ $regexps->{header} = qr/
   $SP?              # maybe one leading space
   (?<line> ={2,6} ) # header marks
   \s*               # leading spaces
-  (?<header> .+ )   # header text (at least one non-empty character, but least small piece)
+  (?<header> \S.+ ) # header text (at least one non-empty character, but least small piece)
   $SP*              # trailing spaces
   (?: \g{line} )    # header mark (exactly same as above)
   (?: $EOL | $)     # newline or file end