Fixed incorrect signature for some UTF characters on POST request encoding.

Hi,

This is related to issue #15 reported by @y8.
From what I have understood, the problem occurs with some multibytes UTF characters. Since the utf8_post_escape function needs to escape only some ASCII characters I just added a condition to ignore escaping on multibytes characters.
Hope this is correct (btw it's the first time I look at an Objective C project, so I did the simplest fix I could think of considering my lack of knowledge here, I'm sure a more elegant solution could be put instead).

Laurent.