charset: reintroduce support for CP125x encodings

In 8eb6e2e, charset handling was
refactored to make use of the ianaindex package. A number of charsets
present in the ianaindex package were removed from being explicitly
mentioned in go-message, leaving only a "quirks table." While nearly all
of the removed charsets are handled by the ianaindex package, support
for three charsets which were previously handled by go-message was
dropped: cp1250, cp1251, and cp1252.

charset/charset.go

@@ -35,6 +35,9 @@ var charsets = map[string]encoding.Encoding{
 
 	"ansi_x3.110-1983": charmap.ISO8859_1,     // see RFC 1345 page 62, mostly superset of ISO 8859-1
 	"gb2312":           simplifiedchinese.GBK, // GBK is a superset of HZGB2312
+	"cp1250":           charmap.Windows1250,
+	"cp1251":           charmap.Windows1251,
+	"cp1252":           charmap.Windows1252,
 }
 
 func init() {