Fixed (Dutch): Add space after 'duizend' when followed by another word

[cmartinez32]

Fixes #628 by adding space after "duizend"

Changes proposed in this pull request:

• Changed elif statement in merge method so it triggers at values =>10^3 instead of values => 10^6. Words with values equal to or greater than 10^3 such as "duizend" and "miljoen" and so forth, will have a space after them as long as they are not the last word in the string.
• Added unit tests covering this spacing behavior for cardinal numbers and fixed existing tests to require a space after "duizend" (only when "duizend" is not the last word).

Status

• READY
• HOLD
• WIP (Work-In-Progress)

How to verify this change

Test with num2words(1001, lang="nl") and ensure that there is space after "duizend". Can use any number n as long as n>1000 and has digits>0 in the hundreds, tens, or ones positions (or equivalently, where "duizend" is not the last word).
Run python -m unittest tests.test_nl to verify Dutch language tests pass.

Additional notes

Justification for this change:

1. https://woordenlijst.org/zoeken/?leidraad=6-9 (linked by @hiddefolkertsma in [Dutch] "duizend" is not followed by a space #628)
2. https://en.wikibooks.org/wiki/Dutch/Vocabulary/The_numbers
3. https://taaladvies.net/aaneenschrijven-van-telwoorden-algemeen/