[FIX] specs of exception not raised: now fails if the underylying code t...

I noticed that some of the specs (that check for exception not raised) are not *really* testing that. It passes simply because the below code happens to throw NoMethodError (which is *not* a TypeError)
```
 GuessHtmlEncoding.encode(nil).should_not raise_error(TypeError)
```

To understand what I mean, try changing the code to:
```
 GuessHtmlEncoding.encode(nil).should raise_error(NoMethodError)
```
It should be done like so:
```
lambda { GuessHtmlEncoding.encode(nil) }.should_not raise_error 
```
Also, it should not raise *any* type of Error, not just TypeError