Fix propertization for sequences of mappings with literal block

[strutt]

The syntax highlighting is broken for lists of dicts where one of the dictionary elements is a literal block. This small fix handles this case.

example:
  - key1: Correctly highlighted
    key2: |
      Correctly highlighted.
  - key3: |
      Correctly highlighted
    key4: Incorrectly highlighted as part of preceding yaml-literal-block

In submitting this PR I'm noticing that the github markdown rendering is incorrect too ^^^. I double checked with the python parser you get a two element list, each of which is a dict with two entries. e.g:

import yaml
import json

doc = """
example:
  - key1: Correctly highlighted
    key2: |
      Correctly highlighted.
  - key3: |
      Correctly highlighted
    key4: Incorrectly highlighted as part of preceding yaml-literal-block
"""

print(json.dumps(yaml.load(doc, Loader=yaml.CLoader), indent=2))

Prints

{
  "Example": [
    {
      "key1": "Correctly highlighted",
      "key2": "Correctly highlighted.\n"
    },
    {
      "key3": "Correctly highlighted\n",
      "key4": "Incorrectly highlighted as part of preceding yaml-literal-block"
    }
  ]
}