Fix bugs

[camilamaia]

• Fix when vars is declared and used in the same request
  Example:

requests:
  - name: new
    method: post
    vars:
      today: ${{datetime.datetime.utcnow().strftime('%Y-%m-%dT%H:%M:%SZ')}}
    body:
      registered_at: ${today}

Before, the evaluation was not working. We were sending the value as ${today}

• Fix when evaluated value is not string

Example

requests:
  - name: new
    method: post
    path: 20

We were receiving the following error:

rary/Caches/pypoetry/virtualenvs/scanapi-KvDFA3Cq-py3.7/lib/python3.7/site-packages/jinja2/runtime.py", line 545, in __next__
    rv = next(self._iterator)
  File "/Users/camilamaia/workspace/scanapi-org/scanapi/scanapi/tree/endpoint_node.py", line 87, in run
    f"\nError to make request `{request.full_url_path}`. \n{str(e)}\n"
  File "/Users/camilamaia/workspace/scanapi-org/scanapi/scanapi/tree/request_node.py", line 66, in full_url_path
    full_url = join_urls(base_path, path)
  File "/Users/camilamaia/workspace/scanapi-org/scanapi/scanapi/utils.py", line 15, in join_urls
    second_url = second_url.lstrip("/")
AttributeError: 'int' object has no attribute 'lstrip'

[github-actions]

@camilamaia your pull request is missing a changelog!