This adds a new interface for httpu that supports a Context, and uses
that context to set a deadline/timeout and also cancel the request if
the context is canceled. Additionally, add a new method to the SSDP
package that takes a ClientInterfaceCtx.
Updates #55
Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
The previous code was misleading by returning the `err` variable rather than an explicit `nil`. This code path was actually the success path. Functionally it was correct as that specific scoping of `err` could only be `nil` if that line of code was reached.
