making keys filter params usage more obvious to fix MB-4277
making keys filter params usage more obvious to fix MB-4277

The default for keys is now set to an empty array containing
an empty string to suggest the usage of the field. Otherwise
the user may have no knowledge of the format required by the
field.

Change-Id: Icd7577eecef65a60a84090154c2b32d9428f0c3f
Reviewed-on: http://review.couchbase.org/9879
Reviewed-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>

daverigby/ns_server,nimishzynga/ns_server,daverigby/ns_server,membase/ns_server,mschoch/ns_server,pavel-blagodov/ns_server,membase/ns_server,daverigby/ns_server,ceejatec/ns_server,nimishzynga/ns_server,daverigby/ns_server,mschoch/ns_server,membase/ns_server,t3rm1n4l/ns_server,membase/ns_server,ceejatec/ns_server,vzasade/ns_server,ceejatec/ns_server,mschoch/ns_server,mschoch/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,membase/ns_server,mschoch/ns_server,vzasade/ns_server,pavel-blagodov/ns_server,mschoch/ns_server,t3rm1n4l/ns_server,vzasade/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,vzasade/ns_server,ceejatec/ns_server,nimishzynga/ns_server,ceejatec/ns_server,pavel-blagodov/ns_server,daverigby/ns_server,nimishzynga/ns_server,membase/ns_server,t3rm1n4l/ns_server,t3rm1n4l/ns_server,pavel-blagodov/ns_server