fix FromParams bug

[epwalsh]

This was an interesting one. I found this after digging into the test that seemed to fail out of nowhere in this (unrelated) PR: https://github.com/allenai/allennlp-models/pull/175/checks?check_run_id=1499712028. This bug only manifested itself because of a different bug fix regarding the ShardedDatasetReader (#4830).

Basically, the ShardedDatasetReader needs to inspect its **kwargs for the "lazy" parameter. The behavior depends on whether or not "lazy" is present. But the current FromParams logic removes the "lazy" param from kwargs if it's the same as the default value in the superclass.

So this fixes the behavior of FromParams so that parameters in the Params object will ALWAYS be passed to the constructor.

[dirkgr]

I don't have the full context, so I trust that you've thought through this more than I have. Is there any chance this will break other expected behavior?

[epwalsh]

Is there any chance this will break other expected behavior?

I can't be completely certain, but we do have very high test coverage here that covers every edge case I can think of so far.