strings.TrimRight remove one more letter.

I don't know if this is the expected result, but I hope it isn't:

``` go
a := strings.TrimRight("1234t.html", ".html")
fmt.Println(a) // Expected 1234t, get 1234
```

[Playground](http://play.golang.org/p/61K08BW1vL)

Edit (Information about my machine according to the guidelines)
Version: 1.5.1
OS: ArchLinux x86_64
The other things are already answered :smile: 
