Entity Reference field handler: restrict reference by ID only to integer field types and integer values

[vaish]

This resolves regression introduced by PR #241. It builds on the PR #280 but introduces some improvements:

1. It ensures value must be positive integer (decimal numbers are not accepted).
2. It partially restores logic that was introduced in Allow Entity Reference Field Handler to Refernce by ID #241 but removed in Restrict reference by ID to numeric values on integer fields #280: It supports finding the referenced entity by label or by ID but only when that makes sense.

In addition I also removed redundant if code block. Condition in that block is already covered by else section of the previous if block.

Resolves #278