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

**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

Start by reproducing the failure with InventoryServiceIT.java, enabling testUpdateSystemProperty(), then run the listed Maven commands. Trace how the RESTClient processes a CompletionStage response and add coverage for this return type; done means the integration test passes without ResponseProcessingException.

Written by the indexing model from the issue text.