Refactored IsSet to examine keys

This patch refactors the IsSet function to examine the keys in order
to see if a key is set instead of simply checking if a value is nil.
This change is necessary due to the fact that default values via
flag bindings will result in the old logic always being true for
the IsSet function due to a type's default value such as 0 for an
integer or an empty string for a string. While a type's default
value may be preferable when getting the value for a key, it
results in a false positive when determining if a key is actually
set. This change enables users to detect whether a key is set by
only returning a flag's value if it has changed.

viper.go

diff --git a/viper.go b/viper.go
index 9727f48..680b331 100644
--- a/viper.go
+++ b/viper.go
@@ -742,8 +742,21 @@
 // Check to see if the key has been set in any of the data locations
 func IsSet(key string) bool { return v.IsSet(key) }
 func (v *Viper) IsSet(key string) bool {
-	t := v.Get(key)
-	return t != nil
+	path := strings.Split(key, v.keyDelim)
+
+	lcaseKey := strings.ToLower(key)
+	val := v.find(lcaseKey)
+
+	if val == nil {
+		source := v.find(strings.ToLower(path[0]))
+		if source != nil {
+			if reflect.TypeOf(source).Kind() == reflect.Map {
+				val = v.searchMap(cast.ToStringMap(source), path[1:])
+			}
+		}
+	}
+
+	return val != nil
 }
 
 // Have Viper check ENV variables for all

viper_test.go

diff --git a/viper_test.go b/viper_test.go
index d9aae00..aa62235 100644
--- a/viper_test.go
+++ b/viper_test.go
@@ -611,6 +611,17 @@
 	assert.Equal(t, 35, v.Get("age"))
 }
 
+func TestIsSet(t *testing.T) {
+	v := New()
+	v.SetConfigType("yaml")
+	v.ReadConfig(bytes.NewBuffer(yamlExample))
+	assert.True(t, v.IsSet("clothing.jacket"))
+	assert.False(t, v.IsSet("clothing.jackets"))
+	assert.False(t, v.IsSet("helloworld"))
+	v.Set("helloworld", "fubar")
+	assert.True(t, v.IsSet("helloworld"))
+}
+
 func TestDirsSearch(t *testing.T) {
 
 	root, config, cleanup := initDirs(t)