[Bugfix] Single druid extraction dimension error

[Mogball]

If you have a single dimension that is an extraction, e.g.

{
    "outputName": "country_uppercase", 
    "extractionFn": {
      "type": "upper"
    }, 
    "type": "extraction", 
    "dimension": "country", 
    "outputType": "STRING"
}

And order_desc is set to true, the query results in an error because dimension is set to the dictionary value, instead of just the dimension name.

[coveralls]

Coverage remained the same at 71.465% when pulling ba643bb on Mogball:mogball/bugfix/single_druid_dimension into 7987cb7 on apache:master.

[coveralls]

Coverage remained the same at 71.465% when pulling ba643bb on Mogball:mogball/bugfix/single_druid_dimension into 7987cb7 on apache:master.

[coveralls]

Coverage remained the same at 71.465% when pulling ba643bb on Mogball:mogball/bugfix/single_druid_dimension into 7987cb7 on apache:master.