Don't store config values that equal their default config value

If 'foo' has not been set and has a default value of 1, 
config.set('foo', 1) will not store the 1.

If 'foo' has been set to X and has a default value of 1, 
config.set('foo', 1) will remove 'foo' from config

spec/app/config-spec.coffee

@@ -1,12 +1,17 @@
 fsUtils = require 'fs-utils'
 
 describe "Config", ->
-  describe ".get(keyPath) and .set(keyPath, value)", ->
-    it "allows a key path's value to be read and written", ->
+  describe ".get(keyPath)", ->
+    it "allows a key path's value to be read", ->
       expect(config.set("foo.bar.baz", 42)).toBe 42
       expect(config.get("foo.bar.baz")).toBe 42
       expect(config.get("bogus.key.path")).toBeUndefined()
 
+  describe ".set(keyPath, value)", ->
+    it "allows a key path's value to be written", ->
+      expect(config.set("foo.bar.baz", 42)).toBe 42
+      expect(config.get("foo.bar.baz")).toBe 42
+
     it "updates observers and saves when a key path is set", ->
       observeHandler = jasmine.createSpy "observeHandler"
       config.observe "foo.bar.baz", observeHandler
@@ -17,6 +22,17 @@ describe "Config", ->
       expect(config.save).toHaveBeenCalled()
       expect(observeHandler).toHaveBeenCalledWith 42
 
+    describe "when the value equals the default value", ->
+      it "does not store the value", ->
+        config.setDefaults("foo", same: 1, changes: 1)
+        expect(config.settings.foo).toBeUndefined()
+        config.set('foo.same', 1)
+        config.set('foo.changes', 2)
+        expect(config.settings.foo).toEqual {changes: 2}
+
+        config.set('foo.changes', 1)
+        expect(config.settings.foo).toEqual {}
+
   describe ".getPositiveInt(keyPath, defaultValue)", ->
     it "returns the proper current or default value", ->
       config.set('editor.preferredLineLength', 0)

src/app/config.coffee

@@ -127,8 +127,10 @@ class Config
   #
   # Returns the `value`.
   set: (keyPath, value) ->
-    _.setValueForKeyPath(@settings, keyPath, value)
-    @update()
+    if @get(keyPath) != value
+      value = undefined if _.valueForKeyPath(@defaultSettings, keyPath) == value
+      _.setValueForKeyPath(@settings, keyPath, value)
+      @update()
     value
 
   setDefaults: (keyPath, defaults) ->