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]
Fixed line 75, 76
Fixed about_regular_expressions.rb to pass tests

Learned about:
* regular expressions,
* using regex to search a string for matching content,
* meanings of ?, + and * when matching a string.

[#65851756]
Fixed about_regular_expressions.rb to pass tests

Learned about:
* the left most match that wins,
* various character classes,
* dot, that is a shortcut for any non newline char,
* negating a character class with ^ and capitals.

[#65851756]
Fixed about_regular_expressions.rb to pass tests

Learned about anchoring:
* to the start of the string,
* to the end of the string,
* to the start of lines,
* to the end of lines.

[#65851756]
Fixed about_regular_expressions.rb to pass tests

Learned about:
* vertical_pipe_that_means_or,
* #scan method that is like find all,
* #sub that is like find and replace,
* and #gsub that is like find and replace all.

[#65851756]
Fixed about_methods.rb to pass tests

Learned about:
* methods and that they can take arguments with or without parentheses...
* but sometimes missing parentheses are ambiguous,
* default values for method arguments,
* and variable number of arguments.

[#65851758]
Fixed about_methods.rb to pass tests

Learned about:
* explicit and implicit returns for methods,
* self,
* private methods,
* calling private methods in other objects.

[#65851758]
Fixed about_constants.rb to pass tests

Learned about:
* nested and top level constants,
* and how they are referenced.

[#65851762]
Fixed about_constants.rb to pass tests

Learned about:
* constants being inherited from enclosing classes,
* and from parent classes;
* and that nested constant wins over inherited,
* unless the scope is explicitly defined.

[#65851762]
Fixed about_control_statements to pass tests

Learned about/that:
* if, then, else statements,
* nil would be returned if no true condition,
* condition operators.

[#65851764]
Fixed about_control_statements to pass tests

Learned about/that:
* statement modifiers,
* unless statement,
* and unless statement modifier.

[#65851764]
Fixed about_control_statements to pass tests

Learned about/that:
* while statement,
* break statement,
* break statement returns value,
* next statement,
* for statement.

[#65851764]
Merge pull request #6 from mkreyman/65851750_about_hashes_master

Fixed about_hashes.rb to pass all tests

[Delivers #65851750]
Merge pull request #9 from mkreyman/65851756_about_regular_expressions_master

about regular expressions master

[Delivers #65851756]
Merge pull request #10 from mkreyman/65851758_about_methods_master

about methods master

[Delivers #65851758]
Added keyword_arguments
Merge pull request #11 from mkreyman/65851762_about_constants_master

about constants master

[Delivers #65851762]
Merge pull request #12 from mkreyman/65851764_about_control_statements_master

about control statements master

[Delivers #65851764]