Fix a bug that some XML element values are parsed as empty string.

XML element that defined as a type of double is not correctly parsed.
This is because Element.set_value does not recognize the 'double' element type definition.

To reproduce this bug, execute "aws rds describe-reserved-db-instances-offerings" on aws-cli
and see 'FixedPrice' value is empty.
Merge pull request #16 from dai0304/feature/bugfix-double

Fix a bug that some XML element values are parsed as empty string.