ARROW-4632: [Ruby] Add BigDecimal#to_arrow

Author: Kenta Murata <[email protected]>

Closes apache#3709 from mrkn/ruby_bigdecimal_to_arrow and squashes the following commits:

28e7f89 <Kenta Murata> Fix test code following the review comments
0cab272 <Kenta Murata> Add BigDecimal#to_arrow

ruby/red-arrow/lib/arrow/bigdecimal-extension.rb

@@ -0,0 +1,24 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+require "bigdecimal"
+
+class BigDecimal
+  def to_arrow
+    Arrow::Decimal128.new(to_s)
+  end
+end

ruby/red-arrow/lib/arrow/decimal128-array-builder.rb

@@ -15,7 +15,7 @@
 # specific language governing permissions and limitations
 # under the License.
 
-require "bigdecimal"
+require "arrow/bigdecimal-extension"
 
 module Arrow
   class Decimal128ArrayBuilder
@@ -36,7 +36,7 @@ def append_value(value)
       when Float
         value = Decimal128.new(value.to_s)
       when BigDecimal
-        value = Decimal128.new(value.to_s)
+        value = value.to_arrow
       end
       append_value_raw(value)
     end

ruby/red-arrow/test/test-bigdecimal.rb

@@ -0,0 +1,24 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+class BigDecimalTest < Test::Unit::TestCase
+  test("#to_arrow") do
+    arrow_decimal = BigDecimal("3.14").to_arrow
+    assert_equal(Arrow::Decimal128.new("3.14"),
+                 BigDecimal("3.14").to_arrow)
+  end
+end