Serialize Rational numbers to a String

With this, we are more compatible with the common JSON encoders,
including the one on RUby core.

lib/rackstash/fields/abstract_collection.rb

@@ -5,6 +5,8 @@
 # of the MIT license. See the LICENSE.txt file for details.
 
 require 'bigdecimal'
+require 'complex'
+require 'rational'
 require 'pathname'
 require 'uri'
 
@@ -153,7 +155,7 @@ module Rackstash
           # number and no way to recover other than manually inspecting the
           # string with the JSON code itself.
           return value.to_s('F').encode!(Encoding::UTF_8).freeze
-        when ::Complex
+        when ::Complex, ::Rational
           # A complex number can not reliably converted to a float or rational,
           # thus we always transform it to a String
           return utf8_encode(value)

spec/rackstash/fields/abstract_collection_spec.rb

@@ -491,10 +491,11 @@ describe Rackstash::Fields::AbstractCollection do
       expect(normalize(complex)).to be_frozen
     end
 
-    it 'transforms Rational to Float' do
+    it 'transforms Rational to String' do
       rational = Rational(-8, 6)
 
-      expect(normalize(rational)).to be_a Float
+      expect(normalize(rational)).to eql '-4/3'
+      expect(normalize(rational).encoding).to eql Encoding::UTF_8
       expect(normalize(rational)).to be_frozen
     end