NPE when using async from groovy with untyped closure

When trying to issue a post using the async method a NullPointerException is thrown by the JerseyInvocation class. Here is an example groovy script which show the issue. It can be executed right away. All dependencies will be resolved by your groovy installation.

```
@Grapes([
@Grab(group='org.glassfish.jersey.core', module='jersey-client', version='2.11'),
@Grab(group='com.google.guava', module='guava', version='17.0')
])
import javax.ws.rs.client.Client
import javax.ws.rs.client.ClientBuilder
import javax.ws.rs.client.InvocationCallback
import javax.ws.rs.client.WebTarget
import java.util.concurrent.ExecutorService
import java.util.concurrent.Executors
import com.google.common.util.concurrent.JdkFutureAdapters

Client client = ClientBuilder.newBuilder().build()
WebTarget webTarget = client.target('https://api.mailgun.net/v2/samples.mailgun.org/messages')
/**
* Fails to execute request.
*
* here: /org/glassfish/jersey/client/JerseyInvocation.java
* Line: 867
* Reason: ReflectionHelper.getParameterizedTypeArguments(pair) returns null for groovy closure
*/
def call1 = webTarget.request().async().post(null, [
completed: { Object response -> println response },
failed: { Throwable t -> System.err.println(t) }
] as InvocationCallback)

/**
* Executes request (but fails due to authorization issues, not relevant here)
*/
def call2 = webTarget.request().async().post(null, new InvocationCallback() {
@Override
void completed(Object o) {
println o
}

@Override
void failed(Throwable throwable) {
System.err.println(throwable)
}
})

ExecutorService executorService = Executors.newSingleThreadExecutor()
def listenableCall1 = JdkFutureAdapters.listenInPoolThread(call1, executorService)
listenableCall1.addListener({ }, executorService)

def listenableCall2 = JdkFutureAdapters.listenInPoolThread(call2, executorService)
listenableCall2.addListener({
client.close()
executorService.shutdown()
}, executorService)
```
#### Environment
JDK 7
#### Affected Versions
[2.11]