Fix HTTP Authorization header parsing

Was returning status 500 if Authorization: Basic token was long, because
_reserve() did not have enough free space at start of buffer.  The
solution was to _commit(r) before attempting the _reserve_str() calls
for the decoded username and password.

As a separate issue, the status 500 also triggered an assertion failure
because http_request_parse_header() was returning 400 but
response.result_code had already been set to 500.
Refactor MeshMS Java API