Series.replace replaces wrong value
I'm running into an issue with `replace` whereby one of the strings that I am using as a key in a replacement dict is not being replaced with the corresponding value. Here is a screen shot:

![screen shot 2013-10-21 at 8 57 34 am](https://f.cloud.github.com/assets/81476/1372831/c40df93a-3a58-11e3-9420-4f6d03d148be.png)

Notice that the second value of 0.5 is not in the replacement Series, even though the element is in the original. I have checked for whitespace issues. I have also tried using integer values instead of floats, but that did not work either.

[Here is the Series I am using](http://cl.ly/S4WZ)
