`CaptureError` sends stacktrace of `Cause()`

[femaref]

Right now, if your error is the following:

import "github.com/pkg/errors"
...
    return errors.WithStack(err)
...

it will not use that stacktrace. The reason is in client.go:

raven-go/client.go

Lines 710 to 712 in 3033899

	cause := pkgErrors.Cause(err)
	packet := NewPacketWithExtra(err.Error(), extra, append(append(interfaces, client.context.interfaces()...), NewException(cause, GetOrNewStacktrace(cause, 1, 3, client.includePaths)))...)

cause gets used for the Stacktrace generation. However, Cause is called on the result of errors.WithStack, which simply removes that layer, so we don't have a trace anymore.

Is there a reason why Cause is used here? Maybe instead of always taking the Cause, check if the current err has a Stacktrace and cause has not, and use the stack of err?

[femaref]

pr: #215