Improve handing of spaces in input names

[brettflorio]

From a FoxyCart user:

<input id='event-participant'
type="text" name="Participant Name" value="" placeholder="Enter
participant's name" class='uk-width-1-1 uk-form-large'
data-parsley-required/>

Fails to add to the cart using the HMAC script as it's trying to hash 'Participant_Name'. The docs mentioned this would happen, just trying to change it or make it a configurable option. Not world-ending, but would make the client smile.