Fix JSON.parse error for client HEAD requests

On the client version, when performing a HEAD request to a JSON resource, superagent tries to parse the res.text with JSON.parse. However, res.text is an empty string and this causes JSON.parse to error with "SyntaxError: JSON Parse error: Unexpected EOF". This patch will set the text to null for HEAD requests, because JSON.parse(null) will not error and simply return null.