JsonNumber::isIntegral did not have the same semantic meaning as in json-schema

unsure if it is a bug in json-p spec or here.

in json-schema 0.0 or 1.0 are valid integer values, but not for JsonNumber::isIntegral.

instead of
```java
public boolean isIntegral() {
  bigdecimal().scale() == 0
}
```
it should be
```java
public boolean isIntegral() {
  bigdecimal().stripTrailingZeros().scale() <= 0
}
```