User-Agent is a special header. If it is set on the Client request, like in the below sample, it will be honored by the framework.
``` 
Response response = target().path(ROOT_PATH).request("text/plain").header("User-Agent", "App Client 2.5-SNAPSHOT").get();
```
The request log shows.
```
1 > GET http://localhost:9998/helloworld
1 > Accept-Encoding: gzip
1 > Host: localhost:9998
1 > Accept: text/plain
1 > User-Agent: App Client 2.5-SNAPSHOT
```
Here if you set multiple headers. 
```
Response response = target().path(ROOT_PATH).request("text/plain").header("User-Agent", "App Client 2.5-SNAPSHOT").header("User-Agent", "App Client 2.4").get();
```
The request log shows.
```
1 > GET http://localhost:9998/helloworld
1 > Accept-Encoding: gzip
1 > Host: localhost:9998
1 > Accept: text/plain
1 > User-Agent: App Client 2.5-SNAPSHOT,App Client 2.4
```
I believe this is expected. Can you provide a better use case?