No unselect available for optional additional contribution on membership contribution pages

Contribution pages with membership sign-ups, and additional recurring or one-time contributions enabled do not provide a perfectly clear way to not make an additional contribution. By default the one-time radio option in the is_recur element is selected as a default, and users just have to leave the amount field blank and continue, but may be confused since it the one-time charge is actively selected.

To recreate:
1. Create a contribution page with memberships, recurring billing, and additional contributions enabled
2. Visit the main contribution page, and see that there is not may to clearly choose NO additional contribution.

Recommended fix:

• in cases where there is a membership sign-up on the page then do not set a default value for is_recur element OR do not set a default when membership sign-up is on the page and the membership sing-up is mandatory (is this case an additional contribution is then definitely optional)
• add an clickable link to reset One-time and Recurring to unselected, OR add a third 'No Thanks' option to the is_recur element in the case where the membership block is present and mandatory

Example: for CRM/Contribute/Form/Contribution/Main.php

// only set default if there is not a separate membership block
if ( !$this->_separateMembershipPayment )

{ $this->_defaults['is_recur'] = 0; }

This issue is related to CRM-3784