Error if org.microshed.testing.jaxrs.RESTClient return CompletionStage

[gkwan-ibm]

Describe the bug
A service provides a REST api return CompletionStage<>.
Used MST RESTClient to test it, got following error:

javax.ws.rs.client.ResponseProcessingException: Problem with reading the data, class java.util.concurrent.CompletionStage, ContentType: application/json.

To Reproduce

• git clone https://github.com/OpenLiberty/guide-microprofile-reactive-messaging-acknowledgment.git
• cd guide-microprofile-reactive-messaging-acknowledgement/finish
• update inventory/src/test/java/it/io/openliberty/guides/inventory/InventoryServiceIT.java to enable the testUpdateSystemProperty() test by uncomment @Test
• mvn -pl models install
• cd inventory
• mvn verify
• output will be

[ERROR] Tests run: 2, Failures: 0, Errors: 1, Skipped: 0, Time elapsed: 41.349 s <<< FAILURE! - in it.io.openliberty.guides.inventory.InventoryServiceIT
[ERROR] testGetProperty  Time elapsed: 0.937 s  <<< ERROR!
javax.ws.rs.client.ResponseProcessingException: Problem with reading the data, class java.util.concurrent.CompletionStage, ContentType: application/json.
	at it.io.openliberty.guides.inventory.InventoryServiceIT.testGetProperty(InventoryServiceIT.java:96)
Caused by: javax.json.bind.JsonbException: Internal error: Unexpected char 82 at (line no=1, column no=1, offset=0)
	at it.io.openliberty.guides.inventory.InventoryServiceIT.testGetProperty(InventoryServiceIT.java:96)
Caused by: javax.json.stream.JsonParsingException: Unexpected char 82 at (line no=1, column no=1, offset=0)
	at it.io.openliberty.guides.inventory.InventoryServiceIT.testGetProperty(InventoryServiceIT.java:96)

Expected behavior
No error and test can be passed

System information:

• OS: Linux, Mac
• Java Version: 8
• MicroShed Testing Version: 0.9

Additional context
MicroProfile rest client supports to return CompletionStage