Fix #734: Shields update on first refresh of an upgraded http page

[kylehickinson]

Seems to be due to a WebKit bug where WKNavigationAction.request.mainDocumentURL is not also upgraded to https, therefore on the first refresh url has https and mainDocumentURL has http. These two properties become aligned once the page is refreshed once.

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• My patch or PR title has a standard commit message that looks like Fix #123: This fixes the shattered coffee cup! (or No Bug: <message> if no relevant ticket)
• Unit Tests are updated to cover new or changed functionality
• User-facing strings use NSLocalizableString()
• New files have MPL-2.0 license header.

Test Plan:

Follow #734

Reviewer Checklist:

• PR is linked to an issue via Zenhub.
• Issues are assigned to at least one epic.
• Issues include necessary QA labels:
  • QA/(Yes|No)
  • release-notes/(include|exclude)
  • bug / enhancement
• Necessary security reviews have taken place.
• Adequate test coverage exists to prevent regressions.
• Adaquate test plan exists for QA to validate (if applicable)