Fix a case where `ripple::Expected` returned a json array, not a value.
The problem was that `Expected` invoked the wrong constructor for the
expected type, which resulted in a constructor that took multiple
arguments being interpreted as an array.
A proposed fix was provided by @godexsoft, which involved a minor
adjustment to three constructors that replaces the use of curly braces
with parentheses. This makes `Expected` usable for
[Clio](https://github.com/XRPLF/clio).
A unit test is also included to ensure that the issue doesn't occur
again in the future.