Strip leading newline char from textarea fields

The legislation and other_evidence fields prepend
a newline character due to a Rails oddity.

Strip these from the string before we make the
request to the Need API.
Send all Maslow-provided fields on a create or update request

When we create or update a need, make sure we send
all the fields which Maslow knows about with the
request. This is so that fields which have been
blanked by the user will still be sent in the
request to the Need API.

As this adds some extra cruft to the request stubs
in the tests, I've added a blank_need_request method
to return an empty need, on which present attributes
can be merged for individual test cases.
Add comment before the newline character stripping
Add impact and currently_met to the blank request test helper
Include nil values in the hash

This also refactors the as_json method so that we
don't duplicate the loops to do almost the same
thing
Merge pull request #26 from alphagov/send-blank-fields-on-update

Send all the fields in the Need API request on create and update