[drone-cli][drone-yaml][bug] drone convert command is faied to parse .drone.yml

Drone Support
1

drone convert command is failed to parse .drone.yml when “matrix”'s “include” or “exclude” is used at the step condition.

drone-cli’s version: 1.1.2

$ drone --version
drone version 1.1.2

Failure case 1

.drone.yml

---
pipeline:
  test:
    image: alpine:3.9.4
    commands:
    - echo ${NAME}
    when:
      matrix:
        include:
          NAME: main
matrix:
  NAME:
  - main
  - foo

$ drone convert
yaml: unmarshal errors:
  line 7: cannot unmarshal !!map into string

Failure case 2

.drone.yml

---
pipeline:
  test:
    image: alpine:3.9.4
    commands:
    - echo ${NAME}
    when:
      matrix:
        exclude:
          NAME: main
matrix:
  NAME:
  - main
  - foo

$ drone convert
yaml: unmarshal errors:
  line 7: cannot unmarshal !!map into string

Success case

.drone.yml

---
pipeline:
  test:
    image: alpine:3.9.4
    commands:
    - echo ${NAME}
    when:
      matrix:
        NAME: main
matrix:
  NAME:
  - main
  - foo

$ drone convert
---
kind: pipeline
name: matrix-1

platform:
  os: linux
  arch: amd64

steps:
- name: test
  pull: default
  image: alpine:3.9.4
  commands:
  - echo main

---
kind: pipeline
name: matrix-2

platform:
  os: linux
  arch: amd64

...

The type of Constraints.Matrix is map[string]string, so it is failed to unmarshal the step condtion such as

    when:
      matrix:
        exclude:
          NAME: main

I try to fix this bug.
It seems to work well.

https://github.com/drone/drone-yaml/compare/master...suzuki-shunsuke:fix/fix-matrix-condition?expand=1

Best regards.