Event::DocType skip initial whitespaces

[BlueGreenMagick]

Document that the whitespace right after <!DOCTYPE is included when reading, and should be included when writing.

I personally think it may be better to exclude the first whitespace from DocType, but that might be a breaking change, so I added a line to documentation instead.

[Mingun]

I think it is better to fix such kinds of problems while we at the 0.x release cycle which is intended for such fixes. Breaking changes is allowed (and expected) between 0.x releases.

[tafia]

I agree to change the behavior if people feel confident this is the best choice. I am not using DOCTYPE much and I am fine having few breaking changes because the next version is already breaking few things.
Best to mention it in the Changelog as well.

[BlueGreenMagick]

Done!

Changes:

• When parsing, skips initial whitespaces after "!DocType". Creates BytesText("") if there is only only whitespace after !DocType even though it's illegal according to the docs.
• When writing, put a single whitespace between !DocType and BytesText content.
• Enabled test_doctype test for serialized feature.

[tafia]

Thanks!