MISSING_TO_STRING_OVERRIDE with invalid field

[jsotuyod]

The folllowing class is reported as MISSING_TO_STRING_OVERRIDE

public class FSTSerializationStrategy implements SerializationStrategy {
    private final FSTConfiguration config;
        ...

This shouldn't happen as:

1. FSTConfiguration doesn't override toString
2. FSTConfiguration is a library class, and therefore, we can't add the missing toString to it.

The current detector should ignore fields that are not part of the application being analyzed, and do not override toString.

Possibly, the first condition can be tested using AnalysisContext.isApplicationClass(), though I'm not sure.