Return error on MarshalJSON of status Undefined

Previously "undefined" was returned as a value. While this is a
valid JavaScript value, it is not valid JSON.

int2.go

@@ -202,7 +202,7 @@ func (src *Int2) MarshalJSON() ([]byte, error) {
 	case Null:
 		return []byte("null"), nil
 	case Undefined:
-		return []byte("undefined"), nil
+		return nil, errUndefined
 	}
 
 	return nil, errBadStatus

int4.go

@@ -193,7 +193,7 @@ func (src *Int4) MarshalJSON() ([]byte, error) {
 	case Null:
 		return []byte("null"), nil
 	case Undefined:
-		return []byte("undefined"), nil
+		return nil, errUndefined
 	}
 
 	return nil, errBadStatus

int8.go

@@ -179,7 +179,7 @@ func (src *Int8) MarshalJSON() ([]byte, error) {
 	case Null:
 		return []byte("null"), nil
 	case Undefined:
-		return []byte("undefined"), nil
+		return nil, errUndefined
 	}
 
 	return nil, errBadStatus

text.go

@@ -144,7 +144,7 @@ func (src *Text) MarshalJSON() ([]byte, error) {
 	case Null:
 		return []byte("null"), nil
 	case Undefined:
-		return []byte("undefined"), nil
+		return nil, errUndefined
 	}
 
 	return nil, errBadStatus