This commit adds support for optional attributes (nil is an allowed
value). Without it, HappyMapper will fail with an error similar to:
TypeError in '#to_xml Address before(:all)'
wrong argument type Float (expected String)
if an attribute is defined with a type other than String, but
is actually nil, when to_xml is called.
The fix is pretty simple - always calling to_s on the value is the
right thing to do for an attribute, because attributes are always
text in XML.