mime/quotedprintable: accept badly encoded bytes

RFC 2045 says:
    An "=" followed by two hexadecimal digits, one or both
    of which are lowercase letters in "abcdef", is formally
    illegal. A robust implementation might choose to
    recognize them as the corresponding uppercase letters.

    https://tools.ietf.org/html/rfc2045#page-22

Change-Id: Ibb4b1e4b8bf4fa65ff895ba486a931d90308bf70
Reviewed-on: https://go-review.googlesource.com/7891Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/mime/quotedprintable/reader.go

@@ -36,6 +36,9 @@ func fromHex(b byte) (byte, error) {
 		return b - '0', nil
 	case b >= 'A' && b <= 'F':
 		return b - 'A' + 10, nil
+	// Accept badly encoded bytes.
+	case b >= 'a' && b <= 'f':
+		return b - 'a' + 10, nil
 	}
 	return 0, fmt.Errorf("quotedprintable: invalid hex byte 0x%02x", b)
 }

src/mime/quotedprintable/reader_test.go

@@ -27,10 +27,10 @@ func TestReader(t *testing.T) {
 		{in: "", want: ""},
 		{in: "foo bar", want: "foo bar"},
 		{in: "foo bar=3D", want: "foo bar="},
+		{in: "foo bar=3d", want: "foo bar="}, // lax.
 		{in: "foo bar=\n", want: "foo bar"},
 		{in: "foo bar\n", want: "foo bar\n"}, // somewhat lax.
 		{in: "foo bar=0", want: "foo bar", err: io.ErrUnexpectedEOF},
-		{in: "foo bar=ab", want: "foo bar", err: "quotedprintable: invalid hex byte 0x61"},
 		{in: "foo bar=0D=0A", want: "foo bar\r\n"},
 		{in: " A B        \r\n C ", want: " A B\r\n C"},
 		{in: " A B =\r\n C ", want: " A B  C"},