Fixed about_true_and_false.rb to pass all tests

Learned about/that:
* true is treated as true,
* and false is treated as false,
* nil is also treated as false,
* everything else is treated as true.

[#65851766]
Fixed about_exceptions.rb to pass tests

Learned about/that:
* exceptions inherit from Exception class,
* rescue clause,
* raising a particular error,
* ensure clause,
* asserting an error is raised.

[#65851770]
Fixed about_keyword_arguments.rb to pass tests

Learned about:
* methods with keyword arguments,
* mandatory arguments.

[#65851760]
Fixed Ruby version
Fixed about_iteration.rb to pass tests

Learned about:
* each is a method on arrays,
* iterations with do and curly brace blocks,
* breaks,
* collect and map methods.

[#65851774]
Fixed about_iteration.rb to pass tests

Learned about:
* select and find_all methods,
* inject method,
* iteration methods work on any collection,

[#65851774]
Fixed about_blocks.rb to pass tests

Learned about:
* yield and methods that can take blocks,
* blocks can be defined with curly brackets or do-end,
* blocks can take arguments,
* methods can call yield multiple times.

[#65851776]
Fixed about_blocks.rb to pass tests

Learned about:
* methods can see if they have been called with a block,
* block can affect variables in the code where they are created,
* blocks can be assigned to variables,
* and passed to methods as arguments,

[#65851776]
Fixed about_sandwich_code.rb to pass tests

Learned about:
* sandwich code,
* abstracting the top and bottom slices.

[#65851778]
Fixed about_classes.rb to pass tests

Learned about:
* instances of classes created with new,
* assignment of instance variables,
* asking for instance variables values with #instance_variable_get,
* use of #instance_eval.

[#65851782]
Fixed about_classes.rb to pass tests

Learned about/that:
* creating accessor methods to return instance variables,
* attr reader will automatically define an accessor,
* attr accessor will automatically define both read and write accessors,
* initialize provides initial values for instance variables,
* args to new must match initialize.

[#65851782]
Fixed about_classes.rb to pass tests

Learned about/that:
* inside a method self refers to the containing object,
* uses of #to_s in string interpolation,
* inspect provides a more complete string version,
* all objects support to_s and inspect.

[#65851782]
Fixed about_open_classes.rb to pass tests

Learned about:
* reopening and modifying existing classes

[#65851784]
Fixed about_scoring_project.rb to pass tests

Learned about:
* a dice game Greed,
* storing counts in a hash.

[#65851780]
Fixed about_modules.rb to pass tests

Learned about:
* modules and how to include them,
* normal methods are available,
* module methods available as well, if module included.

[#65851790]
Fixed about_inheritance.rb to pass tests

Learned about/that:
* subclasses have the parent as an ancestor,
* all classes ultimately inherit from object,
* subclasses inherit behavior from parent class,
* subclasses can also add new behavior.

[#65851788]
Fixed about_inheritance.rb to pass tests

Learned that:
* subclasses can modify existing behavior,
* subclasses can invoke parent behavior via super,
* super does not work cross method.

[#65851788]
Fixed about_scope.rb to pass tests

Learned that:
* you can reference nested classes using the scope operator,
* nested string is not the same as the system string,
* we can use the prefix scope operator to force the global scope.

[#65851792]
Fixed about_scope.rb to pass tests

Learned that:
* constants are defined with an initial uppercase letter,
* class names are just constants,
* constants can be looked up explicitly,
* you can get a list of constants for any class or module.

[#65851792]
Fixed about_to_str.rb to pass tests

 Learned that:
    * #to_s returns a string representation,
    * normally objects cannot be used where strings are expected,
    * #to_str also returns a string representation.

[#65851800]