libs/gst/base/gstbasesrc.c: Update docs. blocksize == 0 now means the default blocksize when working in push based mode.

Original commit message from CVS:
* libs/gst/base/gstbasesrc.c: (gst_base_src_class_init),
(gst_base_src_wait), (gst_base_src_update_length),
(gst_base_src_get_range), (gst_base_src_default_check_get_range),
(gst_base_src_check_get_range), (gst_base_src_pad_check_get_range),
(gst_base_src_loop), (gst_base_src_start),
(gst_base_src_activate_pull):
Update docs.
blocksize == 0 now means the default blocksize when working in push
based mode.
Remove some pointless asserts in _wait function.
Fix offset/length calculations and EOS handling. We can now pull 0
bytes as well, which is allowed.
use _check_get_range() to decide if we can operate in _pull based
mode.
Fix refcounting leak when check_get_range function was not
implemented.
API GstBaseSrc::blocksize range can be 0 too now (default)
* tests/check/elements/filesrc.c: (GST_START_TEST),
(filesrc_suite):
Added check to test _get_range() behaviour.