Accept any MIME type in HTTP requests

Currently when reading text from a URL, the MIME type is set to the
specific format being requested, e.g. application/xml for XML. This
will cause the request to fail with HTTP error 406 if the resource
has a different MIME type, such as text/plain or application/atom+xml.
This commit modifies it to prefer specific types, followed by */* so
that any MIME type is accepted.

I don't have any public URL that exhibits this problem, but it occurred when trying to parse the ATOM feed of TeamCity's built-in NuGet server, as it insists the MIME type of the XML is text/plain.