Add Header option to SObjectDescribeFull to allow for If-Modified-Since

[ahickey-sfdc]

According to https://developer.salesforce.com/docs/atlas.en-us.api_rest.meta/api_rest/sobject_describe_with_ifmodified_header.htm you can choose to retrieve object metadata only if it has changed after a certain date (specified by header).
This is incredibly useful for those with data rate limits and need to implement caching. I think this change would also work if you limited to just the one header type "If-Modified-Since" (and took in a date) but wouldn't be as flexible.