Fix an error handling when key contains any white space

parser.go

@@ -22,6 +22,9 @@ func Parse(tagString string) (map[string]string, error) {
 
 		if inKeyParsing {
 			if unicode.IsSpace(r) {
+				if len(key) > 0 {
+					return nil, errors.New("invalid custom tag syntax: key must not contain any white space, but it contains")
+				}
 				continue
 			}
 

parser_test.go

@@ -98,3 +98,9 @@ func TestShouldRaiseErrorWhenValueIsNotTerminated(t *testing.T) {
 		t.Fatal("expected error has not raised")
 	}
 }
+
+func TestShouldRaiseErrorWhenKeyContainsWhiteSpace(t *testing.T) {
+	if _, err := Parse(`foo:"bar"  bu z:"qux"`); err == nil {
+		t.Fatal("expected error has not raised")
+	}
+}