Add AttrType.Known method

Will allow us to check in pion/turn if a Attribute has
been implemented. We should be returning errors if a
client returns a unknown attribute

Relates to pion/turn#76

attributes.go

@@ -171,6 +171,14 @@ func (t AttrType) String() string {
 	return s
 }
 
+// Known returns true if AttrType is known and implemented
+// by this library.
+func (t AttrType) Known() bool {
+	_, valid := attrNames()[t]
+
+	return valid
+}
+
 // RawAttribute is a Type-Length-Value (TLV) object that
 // can be added to a STUN message. Attributes are divided into two
 // types: comprehension-required and comprehension-optional.  STUN

attributes_test.go

@@ -107,3 +107,12 @@ func TestAttrTypeRange(t *testing.T) {
 		})
 	}
 }
+
+func TestAttrTypeKnown(t *testing.T) {
+	// All Attributes in attrNames should be known
+	for attr := range attrNames() {
+		assert.True(t, attr.Known())
+	}
+
+	assert.False(t, AttrType(0xFFFF).Known()) // Known
+}