The WWW class constructor documentation http://docs.unity3d.com/ScriptReference/WWW-ctor.html has a missing function prototype. The last overridden version of the constructor has "parameters", "description" and "returns" described, however the prototype of the function itself is missing.
It looks like the prototype should be
WWW (string url, byte[] postData, Hashtable headers);

Moreover in previous documentation http://docs.unity3d.com/412/Documentation/ScriptReference/WWW.WWW.html There is a prototype static function
WWW (url : String, postData : byte[], : , headers : string>) : WWW
which should look something like
WWW (string url, byte[] postData, string headers);
but it is not included in the new documentation, so it is unclear if it is a documentation bug or it was actually removed.