Standardize on underscoring query parameters

[TylerPachal]

I was working on a PR to add "dasherized support" for the sort and include fields, in the same way I added it for the filter field in https://github.com/beam-community/jsonapi/pull/282.

While I was doing that, I realized that it already works for include, because the parse_include function performs its own underscoring already, and doesn't need any help from the UnderscoreParameters plug.

This was something that the parse_sort function does not do, which I addressed by changing the UnderscoreParameters plug in https://github.com/beam-community/jsonapi/pull/282.

So now we are in a state where certain query parameters always underscore their values, while others do not:

Query Parameter	Underscored?
fields	Always
filter	Opt-in via replace_query_params
include	Always
sort	Never

I think for version 2.0 of this library:

• All of the query parameters should always be underscored. It should be done by the QueryParser module.
• The UnderscoreParameters module should go back to just doing the params, mostly undoing the work in https://github.com/beam-community/jsonapi/pull/282.

But in the mean time, there should still be a way to underscore the sort query parameter. It looks like our options are:

1. Add the sort underscoring in the same fashion as the filter underscoring by putting it behind the replace_query_params option.
2. Do it all of the time by adding a call to underscore/1 in the QueryParser.parse_filter function.

Let me know what you think is best, and I can prepare the PR.