Move Ruby fielded object creation from generated constructor to ThriftStruct

Move Ruby fielded object creation from generated constructor to ThriftStruct

Rather than the generated code needing to handle simple fielded
creation of objects, the ThriftStruct module constructor is extended to
handle hash arguments. Statements such as

  o = ThriftObject.new :field1 => value1, :field2 => value2, ...

are supported as before, and the string form,

  o = ThriftObject.new "field1" => value1, "field2" => value2, ...

disabled by the previous patch now also works.

Placing this code in the module is also just a cleaner solution.

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@665502 13f79535-47bb-0310-9956-ffa450edef68