It turns out Headers is actually an internal-only API. None of the
user-facing types use it.
Unfortunately, making it unexported also required deleting the doctests,
since doctests can only run against a public interface.
This adds a source field to keep track of upstream errors and allow
backtraces, plus a URL field to indicate what URL an error was
associated with.
The enum variants we used to use for Error are now part of a new
ErrorKind type. For convenience within ureq, ErrorKinds can be turned
into an Error with `.new()` or `.msg("some additional information")`.
Error acts as a builder, so additional information can be added after
initial construction. For instance, we return a DnsFailed error when
name resolution fails. When that error bubbles up to Request's
`do_call`, Request adds the URL.
Fixes#232.
This adds validation of header values on receive, and of both header
names and header values on send. This doesn't change the return
type of set to be a Result, it just validates when the request is
sent. Also removes the section in the README describing handling
of invalid headers, and updates a test that verified acceptance of
non-ASCII headers so that it verifies rejection of them instead.
