Set a default HTTP client for DefaultFetcher in DownloadFile method if none is set (#686)

* if no http client is set on the Fetcher's client field,set the default client

Signed-off-by: Meredith Lancaster <malancas@github.com>

* add test for nil http client scenario

Signed-off-by: Meredith Lancaster <malancas@github.com>

---------

Signed-off-by: Meredith Lancaster <malancas@github.com>

Author: malancas

metadata/fetcher/fetcher.go

@@ -70,6 +70,11 @@ func (d *DefaultFetcher) DownloadFile(urlPath string, maxLength int64, _ time.Du
 		req.Header.Set("User-Agent", d.httpUserAgent)
 	}
 
+	// For backwards compatibility, if the client is nil, use the default client.
+	if d.client == nil {
+		d.client = http.DefaultClient
+	}
+
 	operation := func() ([]byte, error) {
 		// Execute the request.
 		res, err := d.client.Do(req)

metadata/fetcher/fetcher_test.go

@@ -170,3 +170,9 @@ func TestDownloadFile_Retry(t *testing.T) {
 		})
 	}
 }
+
+func TestDownloadFile_NoHTTPClientSet(t *testing.T) {
+	fetcher := DefaultFetcher{}
+	_, err := fetcher.DownloadFile("https://jku.github.io/tuf-demo/metadata/1.root.json", 512000, 0)
+	assert.NoError(t, err)
+}