fix: assign since correctly in changes follower

[ricellis]

PR summary

Assign since correctly when initializing ChangesResultSpliterator

Fixes: #629

Note: An existing issue is required before opening a PR.

PR Checklist

Please make sure that your PR fulfills the following requirements:

• The commit message follows the
  Angular Commit Message Guidelines.
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• New tests
• Build/CI related changes
• Documentation content changes
• Other (please describe)

What is the current behavior?

this.since is used when copying the options for the request, but this.since is only set for the default null cases.

What is the new behavior?

Set this.since to this.options.since() and only then apply the defaults if necessary.

Does this PR introduce a breaking change?

• Yes
• No

Other information