QTS: fix conflict pages/posts in filter_request

[spleen1981]

As the information that query is relevant to a page instead of a post is only included in $wp->matched_query, merge with $query argument must always occur.
If page is identified, name key is removed as it is used to retrieve posts instead.
Posts identification test is moved at the end of the block to avoid other conflicts with more specific tests (e.g. custom post types).

[herrvigg]

Good if this fixes the issue with the page but the code is difficult to read. Many questions. Should be rewritten in a clearer way.

[spleen1981]

code is difficult to read
Should be rewritten in a clearer way.

Agreed, but code reflects the strategy to identify the kind of item processed by current query (at least my understanding), which is actully not very linear, plus the need to preserve possible custom additional query vars.
Couldn't came up with anything clearer, but I'm obviously open to changes as long as it works.

Added some clarifications here 3e954f6