Fix incorrect parse for Excelx with inline rich text

[tomog105]

Summary

I discovered that there is a problem that can not get the contents of inline rich text format cells, since v2.8.0.

Example: richtext_example.xlsx

$ ruby -Ilib -rroo -e 'p Roo::Excelx.new("/Users/ogoke/richtext_example.xlsx").cell("a", 1)'
# result in master
nil
# result in v2.7.1
"Example richtext"

And I examined, A behavior of inline rich text format was changed in Roo::Excelx::SheetDoc#cell_from_xml by 4f9b166.

Therefore, in this PR, a behavior of inline rich text format has been reverted to v2.7.1, and a test case of inline rich text format file is added.

Other Information

This test case file was generated by the following script.

require "bundler/inline"

gemfile do
  source "https://rubygems.org"
  gem "axlsx", "3.0.0.pre"
end

Axlsx::Package.new do |axlsx|
  axlsx.workbook.add_worksheet(name: "richtext") do |sheet|
    rt1 = Axlsx::RichText.new
    rt1.add_run("Example richtext", b: true)

    rt2 = Axlsx::RichText.new
    rt2.add_run("Example", b: true)
    rt2.add_run(" richtext", i: true)

    sheet.add_row [rt1, rt2]
  end
  axlsx.serialize("richtext_example.xlsx")
end

[coveralls]

Coverage increased (+0.03%) to 94.394% when pulling 93ed697 on tomog105:fix_parse_richtext_xlsx into 4ec1104 on roo-rb:master.

[coveralls]

Coverage increased (+0.03%) to 94.394% when pulling 93ed697 on tomog105:fix_parse_richtext_xlsx into 4ec1104 on roo-rb:master.

[coveralls]

Coverage increased (+0.03%) to 94.394% when pulling 93ed697 on tomog105:fix_parse_richtext_xlsx into 4ec1104 on roo-rb:master.