Get CommonMark 0.30 tests to pass for "Fenced code blocks" section

[DavidSSL]

This is very similar to #821 and also originates from #90.

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

FSharp.Formatting/tests/FSharp.Markdown.Tests/CommonMarkSpecTest.fs

Lines 47 to 52 in 09491d1

	[<Test>]
	[<TestCaseSource("getTests")>]
	let ``Commonmark specification`` (_section: string) (markdown: string) (html: string) =
	printfn "Markdown: '%s'" markdown
	(Markdown.ToHtml(markdown, "\n")) |> should equal html

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

The aim would be to ensure that the tests falling under the Fenced code blocks section to pass.

One of the tests is failing as:

Expected string length 26 but was 25. Strings differ at index 9.
Expected: "<p><code> </code>\naaa</p>\n"
But was:  "<p><code></code>\naaa</p>\n"

and this is caused by toCharArray here:

FSharp.Formatting/src/FSharp.Formatting.Markdown/MarkdownParser.fs

Lines 524 to 527 in 09491d1

	let parseSpans (StringPosition.TrimBoth(s, n)) ctx =
	let ctx = { ctx with CurrentRange = Some(n) }
	parseChars [] (s.ToCharArray() |> List.ofArray) ctx |> List.ofSeq

which is "trimming" the whitespace character.