Adds support for nested elements named 'class'

[Confusion]

Defining an item named 'class', as in

class Msg
  include HappyMapper

  has_one :'class', String
end

Msg.parse('<msg></msg>')

currently fails with
lib/happymapper.rb:489:in initialize': undefined method attributes' for nil:NilClass (NoMethodError)

The reason is that the .class method of the HappyMapper object is
overridden and all uses of self.class, starting with the one in
initialize, will fail.

The spec modification in this commit demonstrates another aspect of this
problem (when applied without the other changes) and can serve as a guard
against regression.

The fix is to add __class__ as an alias of class. Of course, if anyone ever
needs __class__ as an item, they will run into the same problem.
However, a generic solution adds a lot more complexity and needing __class__
is (much?) less likely than 'class'.

[coveralls]

Pull Request Test Coverage Report for Build 76

• 13 of 13 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.004%) to 98.54%

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Totals
Change from base Build 75:	0.004%
Covered Lines:	405
Relevant Lines:	411

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💛 - Coveralls