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.

profile.go

@@ -3,6 +3,7 @@ package ofxgo
 import (
 	"errors"
 	"github.com/aclindsa/xml"
+	"strings"
 )
 
 // ProfileRequest represents a request for a server to provide a profile of its
@@ -126,6 +127,35 @@ func (msl *MessageSetList) UnmarshalXML(d *xml.Decoder, start xml.StartElement)
 	}
 }
 
+// MarshalXML handles marshalling a MessageSetList element to an XML string
+func (msl *MessageSetList) MarshalXML(e *xml.Encoder, start xml.StartElement) error {
+	messageSetListElement := xml.StartElement{Name: xml.Name{Local: "MSGSETLIST"}}
+	if err := e.EncodeToken(messageSetListElement); err != nil {
+		return err
+	}
+	for _, messageset := range *msl {
+		if !strings.HasSuffix(messageset.Name, "V1") {
+			return errors.New("Expected MessageSet.Name to end with \"V1\"")
+		}
+		messageSetName := strings.TrimSuffix(messageset.Name, "V1")
+		messageSetElement := xml.StartElement{Name: xml.Name{Local: messageSetName}}
+		if err := e.EncodeToken(messageSetElement); err != nil {
+			return err
+		}
+		start := xml.StartElement{Name: xml.Name{Local: messageset.Name}}
+		if err := e.EncodeElement(&messageset, start); err != nil {
+			return err
+		}
+		if err := e.EncodeToken(messageSetElement.End()); err != nil {
+			return err
+		}
+	}
+	if err := e.EncodeToken(messageSetListElement.End()); err != nil {
+		return err
+	}
+	return nil
+}
+
 // ProfileResponse contains a requested profile of the server's capabilities
 // (which message sets and versions it supports, how to access them, which
 // languages and which types of synchronization they support, etc.). Note that