public String getValues() {
		StringBuilder values = new StringBuilder();
		for (String column : columnNames) {
			if (values.length() > 0) {
				values.append(", ");
			}
			values.append(":payload[" + column + "]");
		}
		return values.toString();
	}
Transforms a payload to a Map representation based on provided column names.

TODO: Right now it simply supports full payload under a 'payload' key or a JSON string with values stored with keys
corresponding to the column names specified. This could be expanded for additional payload types

@author Thomas Risberg
@author Gary Russell
@since 1.0
extensions/spring-xd-extension-jdbc/src/main/java/org/springframework/xd/jdbc/JdbcMessagePayloadTransformer.java