Delete too early start before progress block registered

[nator333]

Hi, I'm a stranger who started using this library. I appreciate your effort for this! This PR is related to issue #50 to let you guys know about what I found. I'm using tag 4.2.1. I tried deleting tryStartInitialPromiseAndStartIfneeded() from progress(block) function because otherwise progress(float) in Promise may be called before registering the block passed from the progress function. I hope this helps. Thanks for reading!