Markdown.pl: sanitize common "oops" entities

Take a hint from w3m and quietly fix up the six common entities
< > & " '   when they are missing their
trailing ';' provided whatever trailing character is there is not
alphanumeric, an equals sign or a semicolon.

Without this change this case the leading ampersand would have ended
up being escaped to & in these cases which seems likely to be
almost certainly incorrect.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>

Markdown.pl

@@ -3315,9 +3315,16 @@ sub _ProcessURLTag {
 }
 
 
+my $oops_entities;
+BEGIN { $oops_entities = qr/(?:lt|gt|amp|quot|apos|nbsp)/io; }
+
+
 sub _HTMLEncode {
     my $text = shift;
 
+    # Treat these accidents as though they had the needed ';'
+    $text =~ s/&($oops_entities)(?![A-Za-z0-9=;])/&$1;/go;
+
     # Ampersand-encoding based entirely on Nat Irons's Amputator MT plugin:
     #   http://bumppo.net/projects/amputator/
     $text =~ s/&(?!#?[xX]?(?:[0-9a-fA-F]+|\w+);)/&/g;
@@ -3335,6 +3342,9 @@ sub _HTMLEncode {
 sub _EncodeAmps {
     my $text = shift;
 
+    # Treat these accidents as though they had the needed ';'
+    $text =~ s/&($oops_entities)(?![A-Za-z0-9=;])/&$1;/go;
+
     # Ampersand-encoding based entirely on Nat Irons's Amputator MT plugin:
     #   http://bumppo.net/projects/amputator/
     $text =~ s/&(?!#?[xX]?(?:[0-9a-fA-F]+|\w+);)/&/g;
@@ -3348,6 +3358,9 @@ sub _EncodeAmpsAndAngles {
 
     my $text = shift;
 
+    # Treat these accidents as though they had the needed ';'
+    $text =~ s/&($oops_entities)(?![A-Za-z0-9=;])/&$1;/go;
+
     # Ampersand-encoding based entirely on Nat Irons's Amputator MT plugin:
     #   http://bumppo.net/projects/amputator/
     $text =~ s/&(?!#?[xX]?(?:[0-9a-fA-F]+|\w+);)/&/g;