Bug 21835: Fix display of request ID

Bug 21835: Fix display of request ID

We now populate the id_prefix property in the API response. It turns out
that explicitly casting to a string wasn't necessary as JS does "the
right thing" when concatenating a string and an integer

Test plan:

- Before applying the patch view the ILL requests table
- TEST: Observe that the Request number column displays as NaN
- Apply the patch
- Refresh the page
- TEST: Observe that the request number now displays correctly
- Add the following block to your koha-conf.xml:

<branch>
    <code>**The code of a branch that has an ILL request attached to
    it**</code>
    <prefix>YAY</prefix>
</branch>

- Refresh the page
- TEST: Observe the request made on the branch that you specified in the
config are now prefixed with "YAY-"

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>