Container::has() should return false for un-mapped interfaces/abstract classes

This is a fix for #168 and https://github.com/mnapoli/PHP-DI-ZF2/pull/6.

`Container::has()` currently returns true for any interface or abstract class. It should return false.

For example, if PHP-DI work with another container X:

- container X can create `FooInterface` because the interface was mapped to an implementation
- PHP-DI has no configuration for `FooInterface`

if container X is called *after* PHP-DI, then PHP-DI will return true for `Container::has()` and `Container::get()` will be called, resulting in an exception. By returning false, that ensures that container X gets called instead.