rename tag_name for frames

I don't believe #293 is the correct fix.

Frame#tag_name and IFrame#tag_name are only there to differentiate which :tag_name is merged into @selector in  IFrame#locator

Element#tag_name makes a wire call, and this hard codes a string result, and is not the desired consistent behavior.

The real issue is that the private #tag_name in the frame file overrides the public Element#tag_name and just needs to be renamed, not made public