Fix header name to associate log entry & trace (#263)

diff --git a/v2/internal/log.go b/v2/internal/log.go
index e4f14ba..210df59 100644
--- a/v2/internal/log.go
+++ b/v2/internal/log.go
@@ -108,7 +108,7 @@
 }
 
 func traceAndSpan(c *context) (string, string) {
-	headers := c.req.Header["Cloud-Trace-Context"]
+	headers := c.req.Header["X-Cloud-Trace-Context"]
 	if len(headers) < 1 {
 		return "", ""
 	}
diff --git a/v2/internal/log_test.go b/v2/internal/log_test.go
index aeecb43..b724153 100644
--- a/v2/internal/log_test.go
+++ b/v2/internal/log_test.go
@@ -437,7 +437,7 @@
 		t.Fatal(err)
 	}
 	for _, h := range headers {
-		req.Header.Add("Cloud-Trace-Context", h)
+		req.Header.Add("X-Cloud-Trace-Context", h)
 	}
 	return fromContext(ContextForTesting(req))
 }