Passing unit tests

:smile:
Parse adjacent asset blocks 

https://github.com/apiaryio/snowcrash/pull/143#discussion-diff-15152659R198

Input:

```
# GET /1
+ response 200

asset

    pre

+ response 404

        Not found
```

Expected result: 

```
OK.
warning: (10)  dangling message-body asset, expected a pre-formatted code block, indent every of its line by 8 spaces or 2 tabs :25:7

{
  "_version": "2.0",
  "metadata": [],
  "name": "",
  "description": "",
  "resourceGroups": [
    {
      "name": "",
      "description": "",
      "resources": [
        {
          "name": "",
          "description": "",
          "uriTemplate": "/1",
          "model": {},
          "parameters": [],
          "actions": [
            {
              "name": "",
              "description": "",
              "method": "GET",
              "parameters": [],
              "examples": [
                {
                  "name": "",
                  "description": "",
                  "requests": [],
                  "responses": [
                    {
                      "name": "200",
                      "description": "",
                      "headers": [],
                      "body": "asset\n\npre\n",
                      "schema": ""
                    },
                    {
                      "name": "404",
                      "description": "",
                      "headers": [],
                      "body": "Not found\n",
                      "schema": ""
                    }
                  ]
                }
              ]
            }
          ]
        }
      ]
    }
  ]
}

```

### Solution

Do not set `lastSectionType` to `UndefinedSectionType` in `SectionParser::parse()` so the Action parser's `processUnexpectedNode()` knows what to do.

E.g.

``` 
if (pd.sectionContext() != UndefinedSectionType)
    lastSectionType = pd.sectionContext();
```