[whatwg/dom] createElementNS() QName/Name confusion (#671)

The spec is not clear about the valid arguments to `Document#createElementNS()`.  It states:
> To validate a qualifiedName, throw an "InvalidCharacterError" DOMException if qualifiedName does not match the Name or QName production. 

But this doesn't explicitly clarify what happens when the `qualifiedName` matches `Name` but not `QName`, for instance the string `foo:bar:bat`.  Since `QName` is defined in term of `Name` anyway, I believe this would be more clearly stated as just `...if qualifiedName does not match the QName production` (eliminating the reference to `Name`).

However, this would change the errors thrown in some corner cases.  According to [the test suite](https://github.com/cscott/web-platform-tests/blob/master/dom/nodes/Document-createElementNS.js), calling `document.createElementNS(null, ":foo")` ought to throw a `NAMESPACE_ERR` (line 24), but `:foo` doesn't match `QName` (prefix must be at least one character in the production), so it ought to throw an `INVALID_CHARACTER_ERROR`. (This assumes that `prefix` will be set to the empty string on line 5 which will trigger the exception on line 6 because the empty string is "non-null".)

On the other hand, `document.createElementNS("http://example.com", ":foo")` ought to throw a `NAMESPACE_ERR` (test case ine 56) yet there is no spec language to justify this: if it failed the "validate qualifiedName" test it ought to throw a INVALID_CHARACTER_ERROR, and steps 5 and 6 state:
>5. If qualifiedName contains a ":" (U+003E), then split the string on it and set prefix to the part before and localName to the part after.
>6. If prefix is non-null and namespace is null, then throw a "NamespaceError" DOMException.

At this point the namespace is non-null and the prefix is the empty string.  There is no language triggering a NamespaceError.

See also https://github.com/web-platform-tests/wpt/pull/12202 which covers some other issues with the test suite for `createElementNS()`, so fixing the test suite might be reasonable.

For best compliance with the existing test suite, the "validate a qualifiedName" test could be written as:

>To validate a qualifiedName, throw an "InvalidCharacterError" DOMException if qualifiedName does not match the Name production.  Otherwise, throw a "NamespaceError" DOMException if qualifiedName does not match the QName production.

That would be my recommended fix.

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