Fix key parsing in line tables (#311)

A bug was reported that indicated that inline tables did not fully support bare keys:
$ echo 'foo = { -bar => "buz"}' | ./tomljson
(1, 9): unexpected token type in inline table: Error

$ echo 'foo = { "whatever" = "buz"}' | ./tomljson
(1, 10): unexpected token type in inline table: String

echo 'foo = { _no = "buz"}' | ./tomljson
(1, 9): unexpected token type in inline table: Error

This change makes a couple of tweaks to to allow for all key variants in inline tables

Fixes: #282

lexer.go

@@ -247,13 +247,13 @@ func (l *tomlLexer) lexRvalue() tomlLexStateFn {
 func (l *tomlLexer) lexLeftCurlyBrace() tomlLexStateFn {
 	l.next()
 	l.emit(tokenLeftCurlyBrace)
-	return l.lexRvalue
+	return l.lexVoid
 }
 
 func (l *tomlLexer) lexRightCurlyBrace() tomlLexStateFn {
 	l.next()
 	l.emit(tokenRightCurlyBrace)
-	return l.lexRvalue
+	return l.lexVoid
 }
 
 func (l *tomlLexer) lexDate() tomlLexStateFn {