Remove call to trim when processing attribute values for directives

The line modified by this pull request calls trim() on attribute values when processing a directive. This means that if your directive expects an attribute value with whitespace it will get removed by angular. For example, when using ng-list, both these instances behave in the same way:

    <input type="text" ng-model="mymodel" ng-list="or">

    <input type="text" ng-model="mymodel" ng-list=" or ">

Which means that when using the separator " or ", an input of "House or Door or Chair" gets incorrectly split into the array ["House","Do","","Chair"].

The change in the request removes the call to trim and includes a test that verifies the above use case works as expected. However, this is potentially a breaking change for applications that rely on whitespace being trimmed.