REST API: Fix multi-type schemas with integer fields.

In [48306] support for multi-typed schemas was improved to first detect the data type of the value before applying further validation. The `integer` data type was detected using the new `rest_is_integer` function. This function used logic, however, that assumed that the value had already passed an `is_numeric` check. This meant that if `integer` and `string` were both acceptable types, the value would always be considered an `integer` causing the later accurate type validation to fail.

This commit fixes the `rest_is_integer` logic to include an `is_numeric` check.

Props rtagliento, TimothyBlynJacobs.
Merges [48881] to the 5.5 branch.
Fixes #51146.
Built from https://develop.svn.wordpress.org/branches/5.5@48883


git-svn-id: https://core.svn.wordpress.org/branches/5.5@48645 1a063a9b-81f0-0310-95a4-ce76da25c4cd