Add the ability to marshal a Response to SGML/XML and test it

This allows for ofxgo to be used to create well-formatted OFX from poor
OFX, or even be used to generate OFX from other formats for easier
importing into financial management software.

Test this functionality by adding "round trip" testing to all existing
tests - ensure that responses' content is the same after a round trip of
marshalling and unmarshalling them.

response.go

@@ -369,3 +369,70 @@ func ParseResponse(reader io.Reader) (*Response, error) {
 		}
 	}
 }
+
+// Marshal this Response into its SGML/XML representation held in a bytes.Buffer
+//
+// If error is non-nil, this bytes.Buffer is ready to be sent to an OFX client
+func (or *Response) Marshal() (*bytes.Buffer, error) {
+	var b bytes.Buffer
+
+	// Write the header appropriate to our version
+	writeHeader(&b, or.Version)
+
+	encoder := xml.NewEncoder(&b)
+	encoder.Indent("", "    ")
+
+	ofxElement := xml.StartElement{Name: xml.Name{Local: "OFX"}}
+
+	if err := encoder.EncodeToken(ofxElement); err != nil {
+		return nil, err
+	}
+
+	if ok, err := or.Signon.Valid(or.Version); !ok {
+		return nil, err
+	}
+	signonMsgSet := xml.StartElement{Name: xml.Name{Local: SignonRs.String()}}
+	if err := encoder.EncodeToken(signonMsgSet); err != nil {
+		return nil, err
+	}
+	if err := encoder.Encode(&or.Signon); err != nil {
+		return nil, err
+	}
+	if err := encoder.EncodeToken(signonMsgSet.End()); err != nil {
+		return nil, err
+	}
+
+	messageSets := []struct {
+		Messages []Message
+		Type     messageType
+	}{
+		{or.Signup, SignupRs},
+		{or.Bank, BankRs},
+		{or.CreditCard, CreditCardRs},
+		{or.Loan, LoanRs},
+		{or.InvStmt, InvStmtRs},
+		{or.InterXfer, InterXferRs},
+		{or.WireXfer, WireXferRs},
+		{or.Billpay, BillpayRs},
+		{or.Email, EmailRs},
+		{or.SecList, SecListRs},
+		{or.PresDir, PresDirRs},
+		{or.PresDlv, PresDlvRs},
+		{or.Prof, ProfRs},
+		{or.Image, ImageRs},
+	}
+	for _, set := range messageSets {
+		if err := encodeMessageSet(encoder, set.Messages, set.Type, or.Version); err != nil {
+			return nil, err
+		}
+	}
+
+	if err := encoder.EncodeToken(ofxElement.End()); err != nil {
+		return nil, err
+	}
+
+	if err := encoder.Flush(); err != nil {
+		return nil, err
+	}
+	return &b, nil
+}