Fix Performance/FixedSize cop.

Slightly refactored the offending code to make it more maintainable.

.rubocop_rubyzip.yml

@@ -123,11 +123,6 @@ Naming/UncommunicativeMethodParamName:
 Naming/VariableName:
   Enabled: false
 
-# Offense count: 2
-Performance/FixedSize:
-  Exclude:
-    - 'test/ioextras/abstract_output_stream_test.rb'
-
 # Offense count: 1
 # Cop supports --auto-correct.
 # Configuration parameters: EnforcedStyle.

test/ioextras/abstract_output_stream_test.rb

@@ -30,13 +30,15 @@ def teardown
   end
 
   def test_write
-    count = @output_stream.write('a little string')
-    assert_equal('a little string', @output_stream.buffer)
-    assert_equal('a little string'.length, count)
-
-    count = @output_stream.write('. a little more')
-    assert_equal('a little string. a little more', @output_stream.buffer)
-    assert_equal('. a little more'.length, count)
+    s1 = 'a little string'
+    count = @output_stream.write(s1)
+    assert_equal(s1, @output_stream.buffer)
+    assert_equal(s1.length, count)
+
+    s2 = '. a little more'
+    count = @output_stream.write(s2)
+    assert_equal(s1 + s2, @output_stream.buffer)
+    assert_equal(s2.length, count)
   end
 
   def test_print