Fixed all tests in about_nil.rb

- Learned that nil is an object.
- Learned that calling a method that nil doesn't know about
  would return NoMethodError with "undefined method" message.
  Learned that nil has several methods: #nil?, #to_s, #inspect.

[#65851740]
Fixed about_objects.rb to pass all tests

Learned that:
- everything in Ruby is an object. (as if I didn't know it yet:)
- objects can be converted to strings.
- objects can be inspected.
- objects have different ID's.
- some system objects always have the same ids: false => 0, true => 2, nil => 4.
- ...as well as small integers.
- cloning an object creates a new object with a unique new id.

[#65851744]
Fixed about_array_assignment.rb to pass all tests

Learned about:
- non-paralel and paralel assignments of array elements to variables.
- splat operator that can be used when the number of variables is unknown.
- swapping variable values with parallel assignment.

[#65851748]
Fixed about_hashes.rb to pass all tests

Learned about:
- creating hashes,
- hash literals,
- various ways to access and modify values in hashes,
- testing hash keys,
- combining hashes,
- default values when creating a new hash,
- that a block can be passed as a default value for new hash.

[#65851750]