Auto merge of #1914 - thirtythreeforty:crate-type, r=alexcrichton

Auto merge of #1914 - thirtythreeforty:crate-type, r=alexcrichton

This fixes a problem with manifest parsing that I detailed [here](http://stackoverflow.com/a/32055245/1830736).  Namely, requesting an invalid `crate-type`, as in the following:
```
[lib]
crate-type = ["bad_type"]
```
did not cause an error to be printed, but rather the bad list of types was silently converted to `["rlib"]`.  This was confusing to the user.

Cargo now properly rejects the above snippet.