Behavior of function writePackageDescription

Hello,

I wrote a small program [bnfcCabal](https://github.com/pascalh/BnfcCabal), which generates very simple cabal files.

This simple program shows how I use cabal functions to create cabal files:

``` haskell
import Distribution.PackageDescription
import Distribution.PackageDescription.Parse
import Distribution.Simple

dep :: Dependency
dep = Dependency (PackageName "base") anyVersion

pd :: PackageDescription
pd = emptyPackageDescription { buildDepends = [ dep ] }

run :: String -> IO ()
run cabalVersion = writePackageDescription (cabalVersion++".cabal") pd
```

Until cabal-1.22.0.0 everything worked fine and my generated cabal file contained the dependency. When using version 1.22.0.0 the cabal file does not anymore contain the dependency.

Is this a bug or am I using the functions wrong (or the wrong functions)?

Here are the generated cabal files using cabal version 1.20.0.3 and 1.22.0.0 respectively:

``` haskell
-- cabal-1.20.0.3
name:
cabal-version: -any
license: AllRightsReserved
build-depends:
base -any
```

``` haskell
-- cabal-1.22.0.0
name:
cabal-version: -any
license: UnspecifiedLicense
```

Start with the writePackageDescription call and reproduce the issue using the bnfcCabal example and its PackageDescription value. Compare the generated cabal files under Cabal 1.20.0.3 and 1.22.0.0, then determine whether the changed dependency output is a bug or reflects incorrect API usage. Done means the behavior is fixed or the correct usage is clearly established.

Written by the indexing model from the issue text.