text: Title: Wrong value of default parameter in report
  Body: #### I'm submitting a ... 
    - [x] bug report
    - [ ] feature request
    - [ ] support request => Please do not submit support request here, see note at the top of this template.
  
  #### What is the current behavior?
  
  This code produces wrong parameters values in report.
  
  ```python
  @allure.step("abc")
  def abc(first_param, default_param=10):
      pass
  
  def test_example():
      abc(1, 2)
  ```
  
  ```json
    "steps": [
      {
        "name": "abc",
        "status": "passed",
        "parameters": [
          {
            "name": "first_param",
            "value": "1"
          },
          {
            "name": "default_param",
            "value": "10"
          }
        ],
  ```
  
  #### What is the expected behavior?
  
  I expect to see `default_param` equals 2 instead of 10.
  
  #### Please tell us about your environment:
  
  - Test framework:        pytest@3.5.1
  - Allure adaptor:        allure-pytest@2.3.2b1
  
  
  