Use CultureInfo.InvariantCulture in DateAsText

[DFU398]

Please check if the PR fulfills these requirements

• I made sure that my code builds
• I merged the master into this branch before pushing
• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce?** (check one with "x")

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior? (You can also link to an open issue here)

See #563

What is the new behavior?

closes #563

Does this PR introduce a breaking change? (check one with "x")

• Yes
• No

If this PR contains a breaking change, please describe the impact and migration path for existing applications

DateAsText previously used CultureInfo.CurrentCulture by default, now it uses CultureInfo.InvariantCulture instead, so the formatting of the resulting string is different if no other format provider was given.
If formatting for the current culture is needed, CultureInfo.CurrentCulture now needs to be explicitly specified, for example:

new DateAsText(date, CultureInfo.CurrentCulture);
new DateAsText(date, format, CultureInfo.CurrentCulture);

Other information