fix type conversion in SecretResponse getter method (#67)

Converting the payload using toString() is not an appropriate way to
feed a JSON parser. We now use JSON roundtrip for type mapping and
introduce shortcuts of the type already matches the target type.

CHANGELOG.md

@@ -1,3 +1,9 @@
+## unreleased
+
+### Fix
+* Fixed JSON type conversion in `SecretResponse#get(String, Class)` (#67)
+
+
 ## 1.1.4 (2023-06-15)
 
 ### Fix