CRM-15104 fixed some issues with line items creating participants with the API

When you create a participant using the API, and this participant has a fee_level with multiple price field values, the api creates an incorrect line item. This pull request contains a unit test and a fix.

See https://issues.civicrm.org/jira/browse/CRM-15104
Please change this to `$priceFieldValueDetails = CRM_Utils_Array::explodePadded($params['fee_level']);` which will
a. strip off the extra value separators from the ends
b. work even if the input is already an array