`engine.properties` does not prevent non-string values

[botic]

The following code can lead to an out-of-sync Java system properties with engine.properties:

const engine = require("ringo/engine");
engine.properties.foo = "bar";
engine.properties.foo === java.lang.System.getProperty("foo"); // true
engine.properties.foo = {};
engine.properties.foo === java.lang.System.getProperty("foo"); // false
java.lang.System.getProperty("foo") === null; // true
typeof engine.properties.foo === "object"; // true
java.lang.System.clearProperty("foo"); // throws a java.lang.ClassCastException!

[botic]

Why are there engine.properties anyway? This looks like a relict from early times, where cleaning up wasn't a priority. Ringo has …

1. engine.properties
2. system.properties (see Implements system.properties, fixes #456 #457 )
3. environment (see Deprecate non-standard environment global, introduce system.properties #456 )

Ringo 4 should address this mess. 🧹