Add Accept Header to wrap requests

[WillAyd]

This is a follow on to the original change in b1abfa8. curl also sets request headers to use this value by default, which can be helpful when trying to connect to sites with very strict content negotiation requirements

[dcbaker]

I wonder if we actually want to present a more realistic set of mime types we accept, or at least weight the kinds we expect higher than ones we don't. Though maybe that will get us into trouble with poorly behaved sites? @eli-schwartz what do you think?

[eli-schwartz]

The first thing that leaps to mind is: what happens on download sites that transparently generate metalink manifests if they detect that the download agent supports them?