[iOS] Clear prototype cell on UnevenRows

[rmarinho]

Description of Change

We were disposing our renderers of the prototype cell so we should also just clear our prototypecell

Bugs Fixed

• fixes NRE using TapGestureRecognizer on cell with HasUnevenRows #1567
• fixes GestureRecognizer on cell template causes iOS crash when using ObservableCollection #1904

PR Checklist

• Has tests (if omitted, state reason in description)
• Rebased on top of master at time of PR
• Changes adhere to coding standard
• Consolidate commits as makes sense

[samhouts]

Should this be pointed at 15-5 instead of master? It's on the vCurrent board.

[rmarinho]

build

[StephaneDelcroix]

please make sure one of your commit says "fixes #xxx" so the squashed merge commit contains it.

[kingces95]

@rmarinho Expect launching iOS gallery and tapping "Go to Test Cases" works but actually fails with exception "Please provide unique tracker + issue number combo" due to existing test test case with matching issue repository and number:

Xamarin.Forms/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue1567.cs

Line 13 in 74cb5c4

[Issue (IssueTracker.Github, 1567, "NRE in NavigationProxy.set_Inner", PlatformAffected.iOS)]

. .

[samhouts]

Please rebase to 15-5. Thanks!

Rebased! Thanks.

[samhouts]

Failing tests. See @kingces95's comment.

[jassmith]

Approved once test is fixed :)

Fixed