added test for empty suffix (+1 squashed commit)

Squashed commits:
[25e1926] allow empty subject suffix

lex.go

@@ -942,8 +942,10 @@ func lexPrefixLabel(l *lexer) stateFn {
 	l.next()
 	l.ignore()
 
-	if l.peek() == '#' {
+	p := l.peek()
+	if p == '#' || isWhitespace(p) {
 		// emit empty IRI suffix, easier than dealing with special case in parser
+		// empty suffix is also valid as per https://dvcs.w3.org/hg/rdf/raw-file/default/rdf-turtle/index.html#grammar-production-PN_LOCAL
 		l.emit(tokenIRISuffix)
 		return lexAny
 	}

lex_test.go

@@ -294,6 +294,13 @@ two
 			{tokenIRIAbs, "http://a.example/o"},
 			{tokenEOF, ""}},
 		},
+		{`p:	<http://a.example/p> <http://a.example/o>`, []testToken{
+			{tokenPrefixLabel, "p"},
+			{tokenIRISuffix, ""},
+			{tokenIRIAbs, "http://a.example/p"},
+			{tokenIRIAbs, "http://a.example/o"},
+			{tokenEOF, ""}},
+		},
 		{"@base <http:/a.org/>.", []testToken{
 			{tokenBase, "base"},
 			{tokenIRIAbs, "http:/a.org/"},

rune.go

@@ -13,6 +13,7 @@ var (
 	badIRIRunes    = [...]rune{' ', '<', '"', '{', '}', '|', '^', '`'}
 	badIRIRunesEsc = [...]rune{' ', '<', '"', '{', '}', '|', '^', '`', '>'}
 	okAfterRDFType = [...]rune{' ', '\t', '<', '"', '\''}
+	whitespace     = [...]rune{' ', '\t', '\r', '\n'}
 	pnTab          = []rune{
 		'A', 'Z',
 		'a', 'z',
@@ -105,6 +106,15 @@ func isPnLocalMid(r rune) bool {
 	return check(r, plTab)
 }
 
+func isWhitespace(r rune) bool {
+	for _, w := range whitespace {
+		if r == w {
+			return true
+		}
+	}
+	return false
+}
+
 const (
 	runeError = math.MaxInt32