Support case-insensitve DOCTYPE with htmlparser

Current bs4/builder/_htmlparser.py BeautifulSoupHTMLParser.handle_decl (lines 188-196) matches for "DOCTYPE " or "DOCTYPE" only. Many sites have doctype in lowercase, resulting in <!DOCTYPE doctype html>. Should check data.lower() against "doctype" like html.parser does.

Or simply, the existing first case can actually apply in all situations, since it's a given (being called by html.parser's parse_html_declaration) the first word is doctype. Simply, `data = data[len("DOCTYPE "):]` will always work. No if/elif required.

html.parser for reference: https://github.com/python/cpython/blob/3.8/Lib/html/parser.py#L265