Handle unprefixed attributes in Element#attribute correctly

[makenowjust]

Currently, Element#attrubute handles unprefixed attributes as attributes with the default namespace, and handles the default namespace as nil value. However, it is a mis-interpretation of the specification of XML 1.0.

XML 1.0 specification says:
(https://www.w3.org/TR/xml-names/#defaulting)

The namespace name for an unprefixed attribute name always has no value.

Therefore, we need to distinguish nil and the default namespace. Also, unprefixed attributes should not be handled as attributes with the default namespace.

Note that XML 1.0 specification also says:

Default namespace declarations do not apply directly to attribute names; the interpretation of unprefixed attributes is determined by the element on which they appear.

This means that we can know (programmatically) how to handle unprefixed attributes from the element's namespace. It does not mean that the namespace of an unprefixed attribute becomes the default namespace.

This commit changes Element#attribute behavior correctly and updates tests for it.

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@kou It breaks backward compatibility, so I'm concerned. What do you think?

[makenowjust]

I also updated here to make it clear that the namespace argument takes a namespace URI.

[kou]

I know the specification (and I know the specification confuses many people) and I can understand this proposal.
But I don't want to introduce incompatibility as much as possible.

In this case, it seems that Element#attribute uses a loose approach. For example, it ignores undefined namespace URI. Based on it, the current behavior (the default namespace is ignored) may be acceptable.

How about adding a new strict version of Element#attribute (and deprecating Element#attribute? Is it too aggressive?) instead? It seems that we have Element#text for text node value and Element#get_text for text node. How about Element#get_attribute? But get_XXX isn't Rubyish. Element#text_node/Element#attribute_node may be better?

[makenowjust]

@kou I agree with deprecating Element#attribute. Also, I'm not sure a new strict version is necessary because we have Element#attributes and Attributes#get_attribute.

[kou]

Then how about documenting this instead of changing the current behavior?
Element#attributes and Attributes#get_attribute will be too long as an alternative way. We can revisit an API for this when someone claims this again.

[makenowjust]

In many cases, people want to obtain an attribute value, and don't want an Attribute object. Thus, people should use element.attributes[name], and long elemene.attributes.get_attributes(name) is not a problem. (get_attribute seems redundant to me, but it is another problem.)

[kou]

element.attributes[name]

FYI: We can use element[name]. :-)