Add test for textMapAdapter.Keys

This is not currently used in normal functioning of Inject and Extract,
but might get used in future so give it some testing now.

Test adapted from `open-telemetry/opentelemetry-go/propagation/propagation_test.go`

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

bridge/opentracing/bridge_test.go

@@ -16,6 +16,7 @@ package opentracing
 
 import (
 	"reflect"
+	"sort"
 	"testing"
 
 	ot "github.com/opentracing/opentracing-go"
@@ -24,6 +25,25 @@ import (
 	"go.opentelemetry.io/otel/propagation"
 )
 
+func TestTextMapAdapterKeys(t *testing.T) {
+	carrier := ot.TextMapCarrier{
+		"foo": "bar",
+		"baz": "qux",
+	}
+
+	keys := textMapAdapter{r: carrier}.Keys()
+	sort.Strings(keys)
+	expected := []string{"baz", "foo"}
+	if !reflect.DeepEqual(expected, keys) {
+		t.Errorf("Keys do not match: %#v, %#v", expected, keys)
+	}
+	// Check what happens if we read from a write-capable adaptor.
+	keys = textMapAdapter{w: carrier}.Keys()
+	if keys != nil {
+		t.Errorf("Keys should be nil: %#v", keys)
+	}
+}
+
 func TestMapCarrier(t *testing.T) {
 	carrier := propagation.MapCarrier{}
 	testBridgeWithCarrier(t, carrier)