Fix: find() looks into "overrides" first

overrides (elements added with Set()) should have highest priority,
checked first (was after PFlags).

From README.md:
> Viper uses the following precedence order. Each item takes precedence
> over the item below it:
> • explicit call to Set
> • flag
> • env
> • config
> • key/value store
> • default

+ fix TestBindPFlags() to use a new Viper instance (otherwise, "port" was
  defined in an earlier test function with Set() and took precedence)

viper.go

@@ -759,7 +759,13 @@ func (v *Viper) find(key string) interface{} {
 	// if the requested key is an alias, then return the proper key
 	key = v.realKey(key)
 
-	// PFlag Override first
+	// Set() override first
+	val, exists = v.override[key]
+	if exists {
+		return val
+	}
+
+	// PFlag override next
 	flag, exists := v.pflags[key]
 	if exists && flag.HasChanged() {
 		switch flag.ValueType() {
@@ -787,7 +793,6 @@ func (v *Viper) find(key string) interface{} {
 			return val
 		}
 	}
-
 	envkey, exists := v.env[key]
 	if exists {
 		if val = v.getEnv(envkey); val != "" {
@@ -795,12 +800,13 @@ func (v *Viper) find(key string) interface{} {
 		}
 	}
 
+	// Config file next
 	val, exists = v.config[key]
 	if exists {
 		return val
 	}
 
-	// Test for nested config parameter
+	//   test for nested config parameter
 	if strings.Contains(key, v.keyDelim) {
 		path := strings.Split(key, v.keyDelim)
 
@@ -815,11 +821,13 @@ func (v *Viper) find(key string) interface{} {
 		}
 	}
 
+	// K/V store next
 	val, exists = v.kvstore[key]
 	if exists {
 		return val
 	}
 
+	// Default as last chance
 	val, exists = v.defaults[key]
 	if exists {
 		return val

viper_test.go

@@ -479,6 +479,7 @@ func TestUnmarshal(t *testing.T) {
 }
 
 func TestBindPFlags(t *testing.T) {
+	v := New() // create independent Viper object
 	flagSet := pflag.NewFlagSet("test", pflag.ContinueOnError)
 
 	var testValues = map[string]*string{
@@ -497,7 +498,7 @@ func TestBindPFlags(t *testing.T) {
 		testValues[name] = flagSet.String(name, "", "test")
 	}
 
-	err := BindPFlags(flagSet)
+	err := v.BindPFlags(flagSet)
 	if err != nil {
 		t.Fatalf("error binding flag set, %v", err)
 	}
@@ -508,7 +509,7 @@ func TestBindPFlags(t *testing.T) {
 	})
 
 	for name, expected := range mutatedTestValues {
-		assert.Equal(t, expected, Get(name))
+		assert.Equal(t, expected, v.Get(name))
 	}
 
 }