Support JSON type

Signed-off-by: Takahiro Nakayama <civitaspo@gmail.com>

Author: civitaspo

lib/embulk/output/bigquery/value_converter_factory.rb

@@ -288,6 +288,10 @@ def json_converter
             Proc.new {|val|
               val
             }
+          when 'JSON'
+            Proc.new {|val|
+              val
+            }
           else
             raise NotSupportedType, "cannot take column type #{type} for json column"
           end

test/test_value_converter_factory.rb

@@ -389,6 +389,12 @@ def test_record
           assert_equal nil, converter.call(nil)
           assert_equal({'foo'=>'foo'}, converter.call({'foo'=>'foo'}))
         end
+
+        def test_json
+          converter = ValueConverterFactory.new(SCHEMA_TYPE, 'JSON').create_converter
+          assert_equal nil, converter.call(nil)
+          assert_equal({'foo'=>'foo'}, converter.call({'foo'=>'foo'}))
+        end
       end
 
       def test_strict_false