Fix a bug in the response parser involving multiple members of a structure all having the same xmlname.  This occurs in responses like DescribeInstanceAttribute where the output structure contains an explicit member for each possible attribute but only one attribute is included in the response.  The value of the one returned response was being duplicated to all of the members of the structure.  Fixes #173.
Merge branch 'fix-enum-parsing' of git://github.com/garnaat/botocore into garnaat-fix-enum-parsing