UniValue does not have a constructor for enum values, however the
compiler will decay the enum into an int and select that constructor.
Avoid this compiler magic and clarify the code by explicitly selecting
the int-constructor.

This is needed for the next commit.

As UniValue provides several constructors for integral types, the
compiler is unable to select one if the passed type does not exactly
match. This is unintuitive for developers and forces them to write
verbose and brittle code.

For example, there are many places where an unsigned int is cast to a
signed int. While the cast is safe in practice, it is still needlessly
verbose and confusing as the value can never be negative. In fact it
might even be unsafe if the unsigned value is large enough to map to a
negative signed one.