make UserAgent optional

Go will use a default value if nothing is set, so there's no need to error out
if one isn't specifically set.

If we don't like the Go-default user agent for some reason, we should define
our own reasonable default, rather than mandating that one be specified.

I haven't been able to find a requirement for this anywhere.

client/client.go

@@ -117,7 +117,9 @@ func (c *Client) createRequest(ctx context.Context, method string, url string, m
 		return nil, errors.New("url is missing")
 	}
 
-	mutators = append(mutators, request.NewHeaderMutator("User-Agent", c.userAgent))
+	if c.userAgent != "" {
+		mutators = append(mutators, request.NewHeaderMutator("User-Agent", c.userAgent))
+	}
 
 	var body io.Reader
 	if requestBody != nil {

client/config.go

@@ -28,9 +28,6 @@ func (c *Config) Validate() error {
 	} else if _, err := url.Parse(c.Address); err != nil {
 		return errors.New("address is invalid")
 	}
-	if c.UserAgent == "" {
-		return errors.New("user agent is missing")
-	}
 
 	return nil
 }

client/config_test.go

@@ -89,11 +89,6 @@ var _ = Describe("Config", func() {
 					Expect(cfg.Validate()).To(MatchError("address is invalid"))
 				})
 
-				It("returns an error if the user agent is missing", func() {
-					cfg.UserAgent = ""
-					Expect(cfg.Validate()).To(MatchError("user agent is missing"))
-				})
-
 				It("returns success", func() {
 					Expect(cfg.Validate()).To(Succeed())
 					Expect(cfg.Address).To(Equal(address))