%YAML 1.1 and '---' not preserved

[harleypig]

'%YAML 1.2' causes a panic, but that's the go yaml library's fault.

This may be related to #19 and others referenced there. The header portion of a strict yaml file is not preserved.

$ cat test.yaml ; yq w test.yaml --inplace a.b d ; printf '=%.0s' {1..40} ; echo ; cat test.yaml
%YAML 1.1
---
a:
  b: c
========================================
a:
  b: d