Retain all relevant header information for resources

[nightsh]

As we're not downloading resources, we currently have no way of knowing some basic information about them without hitting each URL.

However, downloading would be a rather costly action, both in time and disk space. But we can probably get the headers info only.

An idea would be to use Scrapy's cache, if possible, but we need to investigate.

Examples of useful headers to fetch for each downloadable file:

• Content-Type
• Content-Length

Acceptance criteria:

• the JSON dumps generated by scrapers contain headers for all resources
• all the scrapers still run with no errors

[nightsh]

ETA: 3h

[nightsh]

Implemented in #87, pending review and merge.