Issue #581 - coerce attribute names to lower case in HTML mode

[davidfei222]

Fix for issue #581. Attribute names will now always be coerced to lower case when attempting to get or set attributes to keep the behavior in line with how cheerio.load() handles attribute names.

[walshyb]

This PR updates Cheerio's .attr() functionality to more closely match jQuery's .attr(), which saves and retrieves attributes by lowercasing the input name.

[rjpatt]

Adjusted a few existing tests to be able to support tests for this fix.

[fb55]

Thanks a lot for this PR! Some notes:

1. The change in the fixture led to a need for quite a few unrelated changes. Please use one of the other fixtures or add a new one.
2. Cheerio has an XML mode for working with XML. XML is case-sensitive, so we don't want to lowercase tag names in XML. You will probably end up with something along the lines of

attrNameToUse = this.options.xmlMode ? attrName : attrName.toLowerCase()

[davidfei222]

@fb55 Apologies for the late response - my colleagues and I have been very busy at work recently. The requested changes have been made - please re-review at your earliest convenience. Thanks!

[CyberFlameGO]

LGTM!