When a block only has a single class and no other attributes,
it is not necessary to wrap the class attribute in curly braces –
the class name can be placed after the opening mark as is.
This will result in bit cleaner output when pandoc is used
as a markdown pretty-printer.
Previously parsing would break if the code block
contained a string of backticks of sufficient length
followed by something other than end of line.
Closes#5304.