Restore original scroll view delegate when updating content VC

[datwelk]

Fixes #113

Test this as follows:

• create a floating panel with content view controller A that has a scroll view alpha with a delegate X (could be viewcontroller A)
• present the panel
• set the floating panel's content view controller to a view controller B with scroll view beta and a delegate Y (could be viewcontroller B)
• set the floating panel's content view controller back to view controller A
• note that the delegate of scroll view alpha is still X. Without this PR, the delegate is a FloatingPanel instance, instead of the original delegate X.

[scenee]

Thank you for your PR! I will try to test and understand this issue.

[datwelk]

Updated my comment with some more details, also in the original issue #113, let me know if anything is unclear.

[scenee]

I got it! Oh yeah, the library doesn't care of #113. Thanks again. I appreciate it.

Then, I'd like you to modify your PR a bit. A scroll view tracking by FloatingPanelController can be one not managed by a content view controller according to the API design. Therefore it's better to add the following condition 😄

            if let scrollView = floatingPanel.scrollView,
                let delegate = floatingPanel.userScrollViewDelegate,
+                vc.view.subviews.contains(scrollView) {

[datwelk]

Good point @scenee, I have now updated the PR to address this concern.

[scenee]

Thanks a lot!