Fix sua browsers type (bsm#96)

sua.go

@@ -3,7 +3,7 @@ package openrtb
 import "encoding/json"
 
 type UserAgent struct {
-	Browsers     BrandVersion    `json:"browsers,omitempty"`     // A browser or similar software component
+	Browsers     []BrandVersion  `json:"browsers,omitempty"`     // A browser or similar software component
 	Platform     BrandVersion    `json:"platform,omitempty"`     // The user agent’s execution platform / OS
 	Mobile       int             `json:"mobile,omitempty"`       // 1 if the agent prefers a "mobile" version of the content, if available, i.e. optimized for small screens or touch input. 0 if the agent prefers the "desktop". Taken from Sec-CH-UAMobile header
 	Architecture string          `json:"architecture,omitempty"` // Device’s major binary architecture, e.g. "x86" or "arm". Taken from the Sec-CH-UA-Arch header