This makes the code more consistent.

The old one was a copy-paste of the setter.

* Test with frozen Thread
* Test with nil value which removes the entry
* Test different key types

* Test with frozen Thread
* Test different key types

The key type behaviour differs from Thread#thread_variable_set, keys
that raise a TypeError when setting result in a nil value when getting.

* Test with frozen Thread
* Test different key types

The key type behaviour is similar to Thread#thread_variable_get?

* Test different key types

This shows the Symbol type being used as the internal type.

…ble?/#thread_variables specs titles