[5.4] Use Instance instead of Deferred Provider

[Hariador]

When calling app()->instance('foo','bar'), if there's a deferred service provider registered for class foo,
the deferred provider gets called and overrides the instance that was just set. This means that if you have

app()->bind('foo','foo); // in a deferred service provider

and you run the following as the first reference to 'foo'

app()->instance('foo','bar');
$instance = app()->make('foo');

$instance = 'foo' instead of 'bar'

Added an additional condition to Application->make to check to see if an instance for that $abstract as already been set and skip loading the deferred provider if it does.

This issue tends to crop up most when writing tests, since the call to app()->instance is likely the first time the container will have to resolve the binding and overriding a binding with a particular instance of a Mock is common.