Added f.cede for async boundry on empty stream

[ankitkumarrain]

Issue Number -#3697

I modified documentation of the merge combinator. I added f.cede for async boundry and filter empty chunks to avoid unnecessary sendchunk calls.

[ankitkumarrain]

Hey @armanbilge and @mpilquist Please review this code , when you have time .

[armanbilge]

Thanks for the PR. Please add a test related to the linked issue.

[ankitkumarrain]

Hi @armanbilge I have added test for the linked issue with a .ignore mark to skip test. Because it actaully did not made a fix to the issue.
After a deep digging in , Stream.merge.repeat hangs because stream.merge.repeat internally creates an infinite chain of bind (Succeeded(()),k) nodes. The viewl function on pull.scala is a @tailrac loop that handles bind (treminal,count) by immediately recursing, It has no IO boundry in between . This pointed me the fiber running in the stream.empty.repeat is struck in a pure JVM loop forever.
Well, About my changes i have added filter(.non Empty) change in merge combinator that avoids unnecessary sendChunk calls for empty chunks in the merge path.
I was looking for preffered approach for the fix in the right direction . Can you please help me ?