Stacktrace the culprit should be the received error not the cause (#45)

Previously, when stacktrace was enabled, the culprit would be set to
the first error that did not implement Cause (as detected by errors.Cause(err)),
this had the affect of not showing any additional context added to
the error.

This was also inconsistent with the same behaviour when stacktrace was
disabled.

This change attempts to unify that behaviour, as well as to not discard
the context added to the errors.

sentry.go

@@ -197,8 +197,7 @@ func (hook *SentryHook) Fire(entry *logrus.Entry) error {
 			if currentStacktrace == nil {
 				currentStacktrace = raven.NewStacktrace(stConfig.Skip, stConfig.Context, stConfig.InAppPrefixes)
 			}
-			err := errors.Cause(err)
-			exc := raven.NewException(err, currentStacktrace)
+			exc := raven.NewException(errors.Cause(err), currentStacktrace)
 			if !stConfig.SendExceptionType {
 				exc.Type = ""
 			}

sentry_test.go

@@ -300,7 +300,7 @@ func TestSentryStacktrace(t *testing.T) {
 		if packet.Exception.Stacktrace != nil {
 			frames = packet.Exception.Stacktrace.Frames
 		}
-		expectedCulprit := "myStacktracerError!"
+		expectedCulprit := "wrapped: myStacktracerError!"
 		if packet.Culprit != expectedCulprit {
 			t.Errorf("Expected culprit of '%s', got '%s'", expectedCulprit, packet.Culprit)
 		}