Don't use a StringIO when encoding data.

When data is encoded to BERT, each individual, encoded result piece is stored inside an Array based Buffer. At the end, each piece is sequentially written out to a StringIO object and the underlying String is returned. Unfortunately, this sequential writing to StringIO causes a lot of growth of the internal String object. By calling `#join` on the Buffer internal Array, Ruby will allocate a single string that can contain the whole result in a single step.

lib/bert/encode.rb

@@ -62,6 +62,10 @@ module BERT
         @buf.each { |x| io.write x }
       end
 
+      def to_s
+        @buf.join("")
+      end
+
       def bytesize
         @buf.map(&:bytesize).inject :+
       end
@@ -74,11 +78,7 @@ module BERT
     end
 
     def self.encode(data)
-      buf = encode_to_buffer data
-      io = StringIO.new
-      io.set_encoding('binary') if io.respond_to?(:set_encoding)
-      buf.write_to io
-      io.string
+      encode_to_buffer(data).to_s
     end
 
     def self.encode_data(data, io)