Remove Last-Modified and If-Unmodified-Since

We can't make any assertions about modification times or dates on these
due to it not incorporating term/meta changes.
"Create a Post" section had code in <p> vs <code>
Fixed quick spacing in code markdown block
Update routes.md
Merge pull request #565 from WP-API/two-routing-compatibility

Add compatibility with v1 routing
Merge pull request #566 from WP-API/two-kill-last-modified

Remove Last-Modified and If-Unmodified-Since
Merge pull request #567 from NikV/develop

Fixing "Create a Post" section code formatting
Merge branch 'develop' into two-response-hypermedia

Conflicts:
	lib/class-wp-json-posts.php