Get CommonMark 0.30 tests to pass for "Tabs" section

[DavidSSL]

Following the discussion in #90, this is in my opinion one of the steps we have to undertake.

When using the CommonMark version 0.30 test cases, the following battery of tests

https://github.com/fsprojects/FSharp.Formatting/blob/09491d141cb3d1bacb1c4b307a924014c90e8428/tests/FSharp.Markdown.Tests/CommonMarkSpecTest.fs#L47-L52

, experience the problem that some of the tests start failing.

The aim would be to ensure that the tests falling under the Tabs section to pass.

In order to do that, we have to:

• Update the commonmark_spec.json to use the latest test cases
• For the test case

  {
    "markdown": ">\t\tfoo\n",
    "html": "<blockquote>\n<pre><code>  foo\n</code></pre>\n</blockquote>\n",
    "example": 6,
    "start_line": 419,
    "end_line": 426,
    "section": "Tabs"
  },

we get:

Expected string length 58 but was 56. Strings differ at index 24.
  Expected: "<blockquote>\n<pre><code>  foo\n</code></pre>\n</blockquote>\n"
  But was:   "<blockquote>\n<pre><code>foo\n</code></pre>\n</blockquote>\n"