Crash when having a `nil` attributedTitle

[dusek]

Hi,

after I upgraded from 6866d4d to ff72143, my app started crashing. I discovered that it is due to me setting attributedTitle of the startDownloadButton to nil, and this code thus initializing an NSAttributedString with a nil string.

I think the solution is quite easy - in [PKDownloadButton updateButton:title:], check if the passed title is nil. If yes, initialize attrTitle itself as nil.

[ryanholden8]

This is addressed in PR #17

See: https://github.com/PavelKatunin/DownloadButton/pull/17/files#diff-3ad36d91b66ed5d3501db1cd749c212cR121