Unit test that demonstrates bug in state update logic

This commit demonstrates a bug in state handling. Every modification involves a read, modify, write. This unit test mocks out just enough to show that a state file with ssl -> true is read correctly, changed, then produces an invalid json file with two entries in the hash with the same key. This is the key issue as reported in facebook#115.

```
[malmond@devvm1846.vll1 ~/local/taste-tester]
/opt/chefdk/embedded/bin/rspec .
F

Failures:

  1) TasteTester::State should serialize changes correctly
     Failure/Error: expect(@buffer.string).to eq('{"ssl":false}')

       expected: "{\"ssl\":false}"
            got: "{\"ssl\":true,\"ssl\":false}"

       (compared using ==)
     # ./spec/taste-tester_spec.rb:20:in `block (2 levels) in <top
     # (required)>'

Finished in 0.01544 seconds (files took 0.27334 seconds to load)
1 example, 1 failure

Failed examples:

rspec ./spec/taste-tester_spec.rb:6 # TasteTester::State should serialize changes correctly
```

Signed-off-by: Matthew Almond <malmond@fb.com>

lib/taste_tester/state.rb

@@ -145,12 +145,9 @@ def merge(vals)
         state = {}
       end
       state.merge!(vals)
-      ff = File.open(
-        TasteTester::Config.ref_file,
-        'w',
-      )
-      ff.write(state.to_json)
-      ff.close
+      File.open(TasteTester::Config.ref_file, 'w') do |ff|
+        ff.write(state.to_json)
+      end
     rescue StandardError => e
       logger.error('Unable to write the reffile')
       logger.debug(e)

spec/taste-tester_spec.rb

@@ -0,0 +1,23 @@
+require "spec_helper"
+require 'taste_tester/state'
+
+describe TasteTester::State do
+
+  it "should serialize changes correctly" do
+    # initializing a state object creates a directory
+    # so let's pretend it already exists.
+    allow(File).to receive(:directory?).and_return(true)
+
+    # original state file says ssl -> true
+    allow(File).to receive(:read).and_return('{"ssl": true}')
+    @s = TasteTester::State.new
+
+    expect(@s.ssl).to eq(true)
+
+    @buffer = StringIO.new()
+    allow(File).to receive(:open).and_yield(@buffer)
+    @s.ssl = false
+    expect(@buffer.string).to eq('{"ssl":false}')
+  end
+
+end