Accept nil password

In development modus we do not send any sms messages and thus we do not have
any environment variables to define our username and password in it. This is
only valid for production and staging environment. Before this, we defined
username and password directly in initializer file.

When initializing as follows

    Mollie::SMS.username   = ENV['MOLLIE_USERNAME']
    Mollie::SMS.password   = ENV['MOLLIE_PASSWORD']

`username` and `password` will be nil. As result of this,

    @password = Digest::MD5.hexdigest(password)

returns an error:

    TypeError: no implicit conversion of nil into String

Therefore I convert any provided password to string. `nil.to_s` will result in
an empty string ('').