entry.bodySize should never be < -1

[UppaJung]

Running the test suite of JSON files and adding checks to expect bodySize to be >= -1 will reveal many instances in which bodySize is < -1. While -1 is a valid value for this field if one does not know the correct body size, the a document never has a body of negative size.

I discovered this issue while experimenting with porting/re-writing chrome-har in typescript and trying to replicate chrome-har's current behavior. Please feel free to compare against the typescript implementation to test the fix, or to adopt the code from it, which is also MIT license. (You're free to take the whole thing, but it's a very big change of coding style to adapt to if you don't like typescript.) Sorry that the first you are hearing from me is me filing bugs, but leaving them unreported seemed worse than the alternative.