sum results in concatenation rather than addition

[jbothma]

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tableschema-sql==0.10.4
tabulate==0.8.2
tabulator==1.19.0
dataflows==0.0.32
datapackage==1.5.1
datapackage-pipelines==2.0.0

when I run a pipeline including the following steps, excluding the string_to_int processor, I get a cast error at the end of the pipeline because the value 100000-15297008027871520 is not an integer. It looks like the values from the csv are strings, and I suspect the sum operation does += resulting in concatenation.

    - run: load
      parameters:
        from: 'http://datastore.openspending.org/b9d2af843f3a7ca223eea07fb608e62a/estimates-of-provincial-expenditure-2018-19-uploaded-2019-05-01t1328/final/data/epre-2018-19.csv'
      cache: true

      # because otherwise string "addition" happens which looks like
      # '7871520' + '9700802' + '-152' + '100000' = '100000-15297008027871520'
    - run: string_to_int
      parameters:
        column_name: 'value'

    - run: filter
      parameters:
        resources: epre-2018-19
        in:
          - finyear: '2018'

    - run: delete_fields
      parameters:
        resources: epre-2018-19
        fields:
          - voteno
          - econ5
          - sprogno
          - subprogramme

    - run: join
      parameters:
        source:
          name: epre-2018-19
          key:
            - "province"
            - "department"
            - "progno"
            - "programme"
            - "fg1"
            - "fg2"
            - "econ1"
            - "econ2"
            - "econ3"
            - "econ4"
            - "finyear"
            - "budget_phase"
          delete: yes
        target:
          name: epre-2018-19
          key: null
        fields:
          "province": {}
          "department": {}
          "progno": {}
          "programme": {}
          "fg1": {}
          "fg2": {}
          "econ1": {}
          "econ2": {}
          "econ3": {}
          "econ4": {}
          "finyear": {}
          "budget_phase": {}
          "value":
            aggregate: sum
        full: true

| ERROR   :ROW: {'Amount Kind': 'Total', 'Budget Phase': 'Main appropriation', 'Department': 'Education', 'Econ1': 'Capital', 'Econ2': 'Payments for capital assets', 'Econ3': 'Machinery and equipment', 'Econ4': 'Other machinery and equipment', 'Financial Year': 2018, 'FunctionGroup1': 'Learning and culture', 'FunctionGroup2': 'Basic education', 'Government': 'Eastern Cape', 'Programme': 'Public Ordinary School Education', 'Programme Number': 2, 'Value': '100000-15297008027871520'}