api: Set appropriate content-type for success/error responses. (#3537)

Golang HTTP client automatically detects content-type but
for S3 clients this content-type might be incorrect or
might misbehave.

For example:

```
Content-Type: text/xml; charset=utf-8
```

Should be
```
Content-Type: application/xml
```

Allow this to be set properly.

cmd/auth-handler.go

@@ -231,5 +231,5 @@ func (a authHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		a.handler.ServeHTTP(w, r)
 		return
 	}
-	writeErrorResponse(w, r, ErrSignatureVersionNotSupported, r.URL.Path)
+	writeErrorResponse(w, ErrSignatureVersionNotSupported, r.URL)
 }