Second compile doesn't clean the error and warning properties

[monvillalon]

The warning and error properties of the stats result
of the previous compilation and merged into the
new stats instead of reflecting the new values

I think it was introduced here: 91fb22f

I solved it in my local copy by changing

        var stats = compilation.getStats().toJson(options)
        cache = _.merge(cache, stats)  // <<< MERGE ARRAYS
        result = JSON.stringify(cache)
        return result

to

        var stats = compilation.getStats().toJson(options)
        cache = _.assign(cache, stats) // <<< DON'T MERGE ARRAYS
        result = JSON.stringify(cache)
        return result

but don't know if this breaks something else. As far as I can deduce, every compilation event
emits the full value of every property even if some are missing, but I might be wrong and
errors and warnings should be handled as a special case

        var stats = compilation.getStats().toJson(options)
        cache = _.merge(cache, stats) 
        // === DON'T MERGE ERRORS AND WARNINGS
        if (stats.errors) cache.errors = stats.errors
        if (stats.warnings) cache.warnings = stats.warnings
        // === DON'T MERGE ERRORS AND WARNINGS
        result = JSON.stringify(cache)
        return result

I'll write a merge request but I first would like to have your input as to what
approach to take.

Thanks

[monvillalon]

Found the related issue for the commit
#13

Because of this, I think option 3, treating the errors and warnings as a special case
is the smarter choice. But further research has to be done if this
happens with the other properties.

[unindented]

Fixed by #21