Allow multiple arguments to Rackstash::Fields::Array#push

We now also support the new alias to `Array#push` called `append` which
will be added to the core `Array` with Ruby 2.5.

lib/rackstash/fields/array.rb

@@ -85,7 +85,6 @@ module Rackstash
         @raw << normalize(value)
         self
       end
-      alias push <<
 
       # @return [::Array] deep-transforms the array into a plain Ruby Array
       def as_json(*)
@@ -160,6 +159,20 @@ module Rackstash
         self
       end
 
+      # Append — Pushes the given object(s) on to the end of this array. All
+      # values will be normalized before being added. This method returns the
+      # array itself, so several appends may be chained together.
+      #
+      # @param values [::Array] a list of values to append at the end of `self`
+      # @param scope [Object, nil] if any of the (deeply-nested) values is a
+      #   proc, it will be called in the instance scope of this object (when
+      #   given).
+      # @return [self]
+      def push(*values, scope: nil)
+        concat(values, scope: scope)
+      end
+      alias append push
+
       private
 
       def implicit(obj)

spec/rackstash/fields/array_spec.rb

@@ -96,15 +96,13 @@ describe Rackstash::Fields::Array do
       expect(array[1]).to eql 'normalized'
     end
 
+    it 'can append only one value' do
+      expect { array.<< 'foo', 'bar' }.to raise_error ArgumentError
+    end
+
     it 'returns the array' do
       expect(array << 'value').to equal array
     end
-
-    it 'can use push as an alias' do
-      expect(array.push 'value').to equal array
-      expect(array[0]).to eql 'value'
-    end
-
   end
 
   describe '#as_json' do
@@ -270,6 +268,27 @@ describe Rackstash::Fields::Array do
     end
   end
 
+  describe '#push' do
+    it 'can append multiple values' do
+      expect(array.push 'value', 'value2').to equal array
+      expect(array[0]).to eql 'value'
+      expect(array[1]).to eql 'value2'
+    end
+
+    it 'appends arrays as is' do
+      value = ['hello']
+      array.push value
+
+      expect(array[0]).to be_a Rackstash::Fields::Array
+      expect(array[0].to_a).to eql value
+    end
+
+    it 'can use append as an alias' do
+      expect(array.append 'foo').to equal array
+      expect(array[0]).to eql 'foo'
+    end
+  end
+
   describe 'Converter' do
     it 'creates a new array' do
       raw = [Time.now, 'foo']