Improve peformance of utils InlineHash

Use the less expensive .to_a when a prefix is not passed.

lib/gitlab/utils/inline_hash.rb

@@ -35,7 +35,12 @@ module Gitlab
       def merge_keys(hash, prefix: nil, connector: '.')
         result = {}
         base_prefix = prefix ? "#{prefix}#{connector}" : ''
-        pairs = hash.map { |key, value| ["#{base_prefix}#{key}", value] }
+        pairs =
+          if prefix
+            hash.map { |key, value| ["#{base_prefix}#{key}", value] }
+          else
+            hash.to_a
+          end
 
         until pairs.empty?
           key, value = pairs.shift

spec/lib/gitlab/utils/safe_inline_hash_spec.rb

@@ -3,7 +3,7 @@
 require 'fast_spec_helper'
 
 describe Gitlab::Utils::SafeInlineHash do
-  context '.merge_keys!' do
+  describe '.merge_keys!' do
     let(:source) { { 'foo' => { 'bar' => 'baz' } } }
     let(:validator) { instance_double(Gitlab::Utils::DeepSize, valid?: valid) }