JerseyInvocation should override toString()

Override toString() with a sensible implementation.
So if you log a given JerseyInvocation object at least the request method and
URI of the contained requestContext is printed.

See issue #4270

Signed-off-by: Thomas Meyer <thomas.mey@web.de>

core-client/src/main/java/org/glassfish/jersey/client/JerseyInvocation.java

@@ -1001,4 +1001,9 @@ private WebApplicationException createExceptionForFamily(final Response response
     ClientRequest request() {
         return requestContext;
     }
+
+    @Override
+    public String toString() {
+        return "JerseyInvocation [" + request().getMethod() + ' ' + request().getUri() + "]";
+    }
 }