Avoid cropping content after rotating reader post detail view.

Fixes #1689 
This is a band-aid patch for this issue, which I think this is really a symptom of larger problems with the layout logic in the ReaderPostView and WPContentView mentioned in #1729 and #1715. 

The problem had to do with when and how media items update their frames which was further confused by an autoresizing mask. This patch changes thing media items are resized correctly, and the height of the postView is correctly calculated after rotation.

#1729 and #1715 are out-of-scope for 4.1 so I think this patch can suffice for the short term.