Snapshot API no longer works without a `skip_head` parameter

[chester89]

Bug Report

What did you do?
Run curl -XPOST http://localhost:9090/api/v1/admin/tsdb/snapshot
What did you expect to see?
No errors, this worked ok in 2.3.2. Adding ?skip_head=false to the query fixed the problem
What did you see instead? Under which circumstances?
{"status":"error","errorType":"unavailable","error":"unable to parse boolean 'skip_head' argument: strconv.ParseBool: parsing \"\": invalid syntax"}
Environment

• System information:
  Windows 7 SP 1

• Prometheus version:
  2.4.2 revision c305ffa

[chester89]

I think it'd be best to add a default value for skip_head parameter

[gouthamve]

Closed in #4674 and out in 2.4.3