Implements update /posts endpoint and its integration tests

[oguzkocer]

Implements update /posts endpoint and its integration tests following established patterns. It provides 100% integration test coverage - as far as I am aware - for individual field updates. I believe /posts is one of the most important endpoints for us, and the extra coverage will provide helpful. I am even thinking about going back to create /posts endpoint and adding more tests for it.

I've changed the timezone of the test site, so that the date & date_gmt values would be different which should help us avoid related false positive tests.

The meta.footnotes return a JSON string that contains a JSON array. I've added a custom parser and serializer for this, so that we can work with strong types. It seems to be working well at the moment, but it's entirely possible that I am missing some conditions which hopefully will pop-up as we add more WordPress versions and test sites.

[jkmassel]

Anything in meta should probably be left as a HashMap<String, String> – I don't think this library will have enough context to parse what's in there (unless it's generated as part of something in core?)

In that case, I wonder if a separate crate would be the way to go – it could contain a second-level set of parsers for common core-level data stored there?

[oguzkocer]

We briefly discussed post.meta with @jkmassel on Slack and decided to tackle it as its own thing at a later time and it is now being tracked in #306.