ScriptEngine API Fixes

[akbertram]

Changes to Renjin's ScriptEngine implementation to bring it closer to correctness with the API documentation and with the reference Javascript implementation.

[akbertram]

@peter-gergely-horvath does the API require that value from get(key) is equal to the original value set?

Renjin's implementation automatically converts scalars like instances of java.lang.String or `java.lang.Number' to an R object (org.renjin.sexp.SEXP), so what you put in does not always equal what you get out.

[Alain-Bearez]

According to the Javadoc:

Retrieves a value set in the state of this engine. The value might be one which was set using setValue or some other value in the state of the ScriptEngine, depending on the implementation.

https://docs.oracle.com/javase/8/docs/api/javax/script/ScriptEngine.html#get-java.lang.String-

This makes sense if you consider as example an implementation for the JavaScript language: while you put a string value for a key in your bindings, the execution of some script might result in the variable to get an array value.

As a consequence, the assertions on lines 165 and 167 have to be rewritten considering the fact that Renjin wraps the given java.lang.String value into an expression of type StringArrayVector.

    Assert.assertEquals(StringVector.valueOf(value), bindingsFromEngine.get(key));

    Assert.assertEquals(StringVector.valueOf(value), bindingsFromContext.get(key));