Method to query content length before exploration

[farshidtz]

The spec should include mechanisms to know the response size before retrieving TD(s) from a device or directory.

For HTTP, this should be as simple as making a HEAD request to get the Content-Length. This is applicable to:

• Self-describing devices. Small consumers should know if they can accommodate the whole TD or should instead make a selection query Peer-to-Peer Queries Endpoint in Producers #132
• Retrieving a single TD from a directory. Same as above.
• Retrieving multiple or all TDs. Consumers should be able to know if the response is too large for them to process and if they should adjust their query. Listing with chunked transfer #145

[mmccool]

• We need to add a "HEAD" request to all endpoints
• Not clear how to add this to the TD; we could do it outside the TD spec (e.g. add an assertion that HEAD should be supported for a certain set of interactions under HTTP, and to include Content-Length)
• How does this interact with pagination? A: should be exactly what we would returned for a GET; just the headers, just no body
• Should raise an issue to consider how to include this in the TD spec

[mmccool]

@farshidtz will add a PR; will just add an assertion