Add PUT method to update variable metadata

[kyao]

Add PUT /metadata/datasets/{dataset_id}/variables/{variable_id} to update existing metadata.

{
    "description": "ASTI collects primary time-series data on agricultural research capacity and spending levels through national survey rounds in over 80 low-and middle-income countries.",
    "tag": [
        DatasetCode:AE",
        "DatasetName:ASTI R&D Indicators: ASTI-Expenditures",
        "Topic:All government and nonprofit agencies involved in agricultural research in over 80 low- and middle-income countries. ...",
        "Email:asti@cgiar.org",
        "DateUpdate:2019-11-11",
        "CompressionFormat:zip",
        "FileType:csv",
        "FileSize:26KB",
        "FileRows:3094",
        "FileLocation:http://fenixservices.fao.org/faostat/static/bulkdownloads/ASTI_Expenditures_E_All_Data_(Normalized).zip"
    ]
}

[kyao]

Done