Fix Gson.newJsonWriter ignoring lenient and HTML-safe setting

[Marcono1234]

Resolves #1452
Supersedes #1451

⚠️ This is a backward incompatible change because writers created by newJsonWriter will now by default produce HTML-safe output (because that is Gson's default setting). However, arguably the previous behavior was incorrect.

[eamonnmcmanus]

This seems reasonable. Most likely people have been affected only by having to call setLenient etc on the returned JsonWriter even though they had already called it on the originating Gson object. I did try this change against the tests of all of Google's internal projects and saw no problems.

I think I would like to make a release before merging this change and #1992, since they are at least potentially breaking changes. And I'd like the OSGi fix from #1981 or an equivalent fix to be in that release. So it might be a little while before these are merged.

[@Marcono1234 could you e-mail me at my @google.com address?]

[Marcono1234]

Most likely people have been affected only by having to call setLenient etc on the returned JsonWriter even though they had already called it on the originating Gson object.

The main problem is the HTML-safe setting: Gson has it enabled by default, but did not propagate it to the JsonWriter. So once this pull request is merged all usage of newJsonWriter will by default produce HTML-safe output (which was not the case before). This should not matter much because it is still valid JSON data, but maybe this causes issues in some situations. Retrofit seems to use that method so it might affect a large (?) number of users; however on the other hand some of them do want HTML-safe output, see also the description of #1452.

So it would be good indeed to have that in a separate release.

[eamonnmcmanus]

The 2.8.9 release has now happened so we can merge this as part of the future 2.9.0.