Path item $refs override inline operations instead of merging

Demonstration API definition

2.0 spec:

swagger: '2.0'
info:
  version: 0.0.0
  title: Path item $ref test

paths:
  /foo:
    get:
      responses:
        200:
          description: OK

  /foo2:
    $ref: '#/paths/~1foo'
    post:
      summary: Create a foo2
      responses:
        201:
          description: Created

3.0 spec:

openapi: 3.0.0
info:
  version: 0.0.0
  title: Path item $ref test

paths:
  /foo:
    get:
      responses:
        '200':
          description: OK

  /foo2:
    $ref: '#/paths/~1foo'
    post:
      summary: Create a foo2
      responses:
        '201':
          description: Created

Expected Behavior

/foo2 displays both GET (added by $ref) and POST (defined inline).

Current Behavior

/foo2 displays GET only.

Context

According to https://github.com/OAI/OpenAPI-Specification/issues/1038#issuecomment-295594246, when a path item contains a $ref, the referenced operation is added (merged) to other operations in this path. Unlike regular JSON references, path item $refs are not supposed to override their sibling elements.

Start by reproducing the issue in Swagger UI with the 2.0 and 3.0.0 API definitions shown in the report, focusing on how the /foo2 path item is resolved. Done means /foo2 displays both the referenced GET operation and its inline POST operation instead of only GET.