SZTextView placeholder text view ignores custom `textContainerInset` value set before `awakeFromNib`

In some circumstances (notable example: in `awakeFromNib` method of `UITableViewCell` loaded from nib, which contains an SZTextView), `textContainerInset` property may be changed on a text view between `initWithCoder` and `awakeFromNib` calls. `exclusionPaths` property, for example, is already processed in this case, but `textContainerInset` is not, thus this inset becomes "out-of-sync" between the "real" text view and placeholder text view. A solution is to set the current value on placeholder text view in `awakeFromNib`.