Issue #58 - Request sample Content-Type header

[minazek]

Issue #58

Aim:

Content-Type header in request sample section needs to match request Content-Type

Solution:

• Add rack gem dependancy because it is used in code
• Use Content-Type instead Accept header in lib/dox/printers/example_request_printer.rb

[akapronik]

During setting app new app with dox, it was discovered that the rexml gem was removed as dependency from rubocop, and other gems that ends up in this error on CI:

LoadError:
  cannot load such file -- rexml/document

So to fix that issue for new apps we added dependency of rexml to the dox in d1cf196

Also, the ruby version was updated in 18c2010