add getparamdefault function

Stdlib currently has `getparam` function which returns parameter value for a resource. The `getparamdefault` function returns **default** value of resource's (or type's) parameter, for example: 
```
Package { provider => aptitude }
package { 'apache2': provider => dpkg }
$p1 = getparam(Package['apache2'], provider) # -> dpkg
$p2 = getparamdefault(Package['apache2'], provider) # -> aptitude
$p3 = getparamdefault(Package, provider) # -> aptitude
```

It may be helpful when developing new classes and defined types, for example:

```
class apache_package {
  if getparamdefault(Package, provider) == 'dpkg' {
    package { 'apache2':   source => '/usr/packages/apache2_2.4.6-2.dpkg'  }
  } else {
    package { 'apache2': }
  }
}
```