Improve consistency of array and array_t with regard to other pytypes

* `array_t(const object &)` now throws on error
* `array_t::ensure()` is intended for casters —- old constructor is
  deprecated
* `array` and `array_t` get default constructors (empty array)
* `array` gets a converting constructor
* `py::isinstance<array_T<T>>()` checks the type (but not flags)

There is only one special thing which must remain: `array_t` gets
its own `type_caster` specialization which uses `ensure` instead
of a simple check.
diff --git a/tests/test_numpy_array.cpp b/tests/test_numpy_array.cpp
index df6377e..14c4c29 100644
--- a/tests/test_numpy_array.cpp
+++ b/tests/test_numpy_array.cpp
@@ -126,4 +126,28 @@
     );
 
     sm.def("function_taking_uint64", [](uint64_t) { });
+
+    sm.def("isinstance_untyped", [](py::object yes, py::object no) {
+        return py::isinstance<py::array>(yes) && !py::isinstance<py::array>(no);
+    });
+
+    sm.def("isinstance_typed", [](py::object o) {
+        return py::isinstance<py::array_t<double>>(o) && !py::isinstance<py::array_t<int>>(o);
+    });
+
+    sm.def("default_constructors", []() {
+        return py::dict(
+            "array"_a=py::array(),
+            "array_t<int32>"_a=py::array_t<std::int32_t>(),
+            "array_t<double>"_a=py::array_t<double>()
+        );
+    });
+
+    sm.def("converting_constructors", [](py::object o) {
+        return py::dict(
+            "array"_a=py::array(o),
+            "array_t<int32>"_a=py::array_t<std::int32_t>(o),
+            "array_t<double>"_a=py::array_t<double>(o)
+        );
+    });
 });