Metadata: Add Content Types To Support

[chaiwa-berian]

The Web API should offer the following content types for metadata:

Content-Type | URL extension | Description
application/xml | .xml | Returns the metadata in XML representation
application/json | .json | Returns the metadata in JSON representation
application/pdf | .pdf | Returns the metadata as a PDF document
application/csv | .csv | Returns the metadata as a CSV file
application/vnd.ms-excel | .xls | .xlsx | Returns the metadata as an Excel workbook