Clean extra tag contents when determining field name

mapstructure.go

diff --git a/mapstructure.go b/mapstructure.go
index 253859f..e7c4155 100644
--- a/mapstructure.go
+++ b/mapstructure.go
@@ -382,6 +382,7 @@
 		fieldName := fieldType.Name
 
 		tagValue := fieldType.Tag.Get(d.config.TagName)
+		tagValue = strings.SplitN(tagValue, ",", 2)[0]
 		if tagValue != "" {
 			fieldName = tagValue
 		}

mapstructure_test.go

diff --git a/mapstructure_test.go b/mapstructure_test.go
index 5535f8e..5f359c2 100644
--- a/mapstructure_test.go
+++ b/mapstructure_test.go
@@ -49,6 +49,7 @@
 }
 
 type Tagged struct {
+	Extra string `mapstructure:"bar,what,what"`
 	Value string `mapstructure:"foo"`
 }
 
@@ -525,6 +526,7 @@
 
 	input := map[string]interface{}{
 		"foo": "bar",
+		"bar": "value",
 	}
 
 	var result Tagged
@@ -536,6 +538,10 @@
 	if result.Value != "bar" {
 		t.Errorf("value should be 'bar', got: %#v", result.Value)
 	}
+
+	if result.Extra != "value" {
+		t.Errorf("extra should be 'value', got: %#v", result.Extra)
+	}
 }
 
 func testSliceInput(t *testing.T, input map[string]interface{}, expected *Slice) {